U.S. patent application number 15/820150 was filed with the patent office on 2018-12-06 for nanofibril materials for highly sensitive and selective sensing of amines.
The applicant listed for this patent is University of Utah Research Foundation. Invention is credited to Yanke Che, Ling Zang.
Application Number | 20180348129 15/820150 |
Document ID | / |
Family ID | 42398027 |
Filed Date | 2018-12-06 |
United States Patent
Application |
20180348129 |
Kind Code |
A1 |
Zang; Ling ; et al. |
December 6, 2018 |
Nanofibril Materials for Highly Sensitive and Selective Sensing of
Amines
Abstract
A sensory material with high sensitivity, selectivity, and
photostability has been developed for vapor probing of organic
amines. The sensory material is a perylene-3,4,9,10-tetracarboxyl
compound having amine binding groups and the following formula
##STR00001## where A and A' are independently chosen from N--R1,
N--R2, and O such that both A and A' are not O, and R1 through R10
are amine binding moieties, solubility enhancing groups, or
hydrogen such that at least one of R1 through R10 is an amine
binding moiety. This perylene compound can be formed into
well-defined nanofibers. Upon deposition onto a substrate, the
entangled nanofibers form a meshlike, highly porous film, which
enables expedient diffusion of gaseous analyte molecules within the
film matrix, leading to a milliseconds response for vapor
sensing.
Inventors: |
Zang; Ling; (Salt Lake City,
UT) ; Che; Yanke; (Salt Lake City, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Utah Research Foundation |
Salt Lake City |
UT |
US |
|
|
Family ID: |
42398027 |
Appl. No.: |
15/820150 |
Filed: |
November 21, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13942219 |
Jul 15, 2013 |
9823193 |
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15820150 |
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12696952 |
Jan 29, 2010 |
8486708 |
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13942219 |
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61148780 |
Jan 30, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 428/298 20150115;
G01N 2021/6432 20130101; Y10T 436/173845 20150115; C09K 11/06
20130101; Y10T 428/2973 20150115; C09K 2211/1011 20130101; C07D
471/06 20130101; G01N 21/6428 20130101; C07D 491/06 20130101 |
International
Class: |
G01N 21/64 20060101
G01N021/64; C09K 11/06 20060101 C09K011/06; C07D 491/06 20060101
C07D491/06; C07D 471/06 20060101 C07D471/06 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with government support under Grants
CHE0641353 and CBET730667 awarded by the National Science
Foundation. The Government has certain rights to this invention.
Claims
1. An amine sensor assembly comprising a porous film of entangled
nanofibers on a substrate, the nanofibers having a stacked
nanofiber structure including a 3,4,9,10-tetracarboxyl perylene
compound having the formula I: ##STR00008## where A and A' are
independently chosen from N--R1, N--R2, and O such that both A and
A' are not O, and R1 through R10 are independently selected from
the group consisting of amine binding moieties, solubility
enhancing groups, and hydrogen such that at least one of R1 through
R10 is an amine binding moiety.
2. The amine sensor assembly of claim 1, wherein A is N--R1 and A'
is O.
3. The amine sensor assembly of claim 2, wherein R1 is a C1 to C13
alkyl.
4. The amine sensor assembly of claim 2, wherein R1 is hexylheptyl,
pentylhexyl, or butylpentyl.
5. The amine sensor assembly of claim 1, wherein A is N--R1 and A'
is N--R2.
6. The amine sensor assembly of claim 5, wherein at least one of R1
and R2 is a C1 to C13 alkyl.
7. The amine sensor assembly of claim 5, wherein at least one of R1
and R2 is selected from the group consisting of hexylheptyl,
pentylhexyl, butylpentyl, COOH, cyclopentyl, cyclohexyl,
cycloheptyl, cyclooctyl, and cyclododecyl.
8. The amine sensor assembly of claim 5, wherein one or two of R3
through R10 is COOH.
9. The amine sensor assembly of claim 1, wherein R4 and R5
collectively form maleic anhydride.
10. The amine sensor assembly of claim 1, wherein the nanofibers
have a diameter from about 10 nm to about 1000 nm.
11. The amine sensor assembly of claim 10, wherein the nanofibers
have a diameter from about 10 nm to about 50 nm
12. The amine sensor assembly of claim 1, wherein the amine-binding
moiety includes an oxygen moiety or an acid.
13. The amine sensor assembly of claim 12, wherein amine-binding
moiety is an anhydride.
14. The amine sensor assembly of claim 12, wherein the
amine-binding moiety is a carboxylic acid.
15. The amine sensor assembly of claim 1, wherein R3-R10 contains
at least one amine-binding moiety or solubility enhancing
group.
16. The amine sensor assembly of claim 1, wherein A is N--R1 and A'
is O and R1 is a branched alkyl or wherein A is N--R1 and A' is
N--R2 and the second amine-binding moiety is an anhydride or
carboxylic acid and the solubility enhancing group is a branched
alkyl.
17. A method of detecting amines in a fluid, comprising: a)
exposing the fluorescent sensor compound of claim 1 to a fluid
sample; and b) displaying a fluorescence change upon exposure of
the nanofiber sensor compound to the fluid sample.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/942,219, filed on Jul. 15, 2013, and issued as U.S. Pat. No.
9,823,193, which is a continuation of U.S. application Ser. No.
12/696,952, filed Jan. 29, 2010, and issued as U.S. Pat. No.
8,486,708, which claims the benefit of U.S. Provisional Application
61/148,780, filed Jan. 30, 2009 which are each incorporated herein
by reference.
FIELD OF THE INVENTION
[0003] This invention relates generally to fluorescent materials,
and the use of such materials for detection of amines. Therefore,
the present invention relates generally to the fields of chemistry
and materials science.
BACKGROUND OF THE INVENTION
[0004] Development of sensors or probes that can be used to detect
the trace vapor of organic amines represents one of the active
research fields in chemistry and materials science, particularly
those related to the emerging nanoscience and nanotechnology.
Volatile amines have been heavily used in various areas ranging
from chemical and pharmaceutical to food industries. Some of the
amines, like hydrazine, have also been used in the military as fuel
additives in rocket and fighter jet propulsion systems. Detecting
these amines with high sensitivity is not only critical to air
pollution monitoring and control but also may provide expedient
ways for quality control of food and even medical diagnosis of
certain types of disease. For example, in diagnosing uremia and
lung cancer, released biogenic amines are commonly used as
biomarkers.
[0005] Although much success has been achieved for detection of
amines in solutions using various types of sensors, the vapor-based
detection of gaseous amines still remains challenging. This
challenge is largely due to the limited availability of sensory
materials that enable vapor detection with both high sensitivity
and selectivity. Fluorescent sensing and probing based on organic
sensory materials represents a unique class of detection techniques
that usually provide a simple, expedient way for chemical detection
and analysis. However, there are not many organic materials
available that are sufficiently fluorescent in the solid state and
suited for use as sensory materials in vapor detection. These
materials may be strongly fluorescent in molecular state in
solutions. Moreover, compared to the more common p-type (i.e.,
electron donating) materials, which are suited for sensing
oxidative reagents like nitro-based compounds, the availability of
n-type organic materials (i.e., electron accepting, and suited for
sensing reducing reagents like amines) is much more limited.
SUMMARY
[0006] In light of the problems and deficiencies noted above,
amines sensor assemblies can include a porous film of entangled
nanofibers on a substrate. The nanofibers can include
3,4,9,10-tetracarboxyl perylene compounds having the formula I:
##STR00002##
[0007] where A and A' are independently chosen from N--R1, N--R2,
and O such that both A and A' are not O, and R1 through R10 are
amine binding moieties, solubility enhancing groups, or hydrogen
such that at least one of R1 through R10 is an amine binding
moiety.
[0008] A nanofiber-based sensor compound can be formed via
synthesis of the underlying perylene compound which is then formed
into the nanofibers. For example, a 3,4,9,10-tetracarboxyl perylene
compound having the Formula I (as previously noted) can be
synthesized. The perylene compound can be self-assembled into
nanofibers via any suitable process such as, but not limited to, a
slow controlled solvent-exchange step, rapid solution dispersion,
phase transfer at the interface between two solvents, sol-gel
processing, direct vaporization of the solvent, or any other
suitable self-assembly methods including the surface assisted
process. The nanofiber fluorescent sensor compound can optionally
be formed into a film of entangled nanofibers by coating the
nanofiber dispersion on a substrate.
[0009] There has thus been outlined, rather broadly, the more
important features of the invention so that the detailed
description thereof that follows may be better understood, and so
that the present contribution to the art may be better appreciated.
Other features of the present invention will become clearer from
the following detailed description of the invention, taken with the
accompanying figures and claims, or may be learned by the practice
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention will become more fully apparent from
the following description and appended claims, taken in conjunction
with the accompanying drawings. Understanding that these drawings
merely depict exemplary embodiments of the present invention and
they are, therefore, not to be considered limiting of its scope. It
will be readily appreciated that the components of the present
invention, as generally described and illustrated in the figures
herein, could be arranged, sized, and designed in a wide variety of
different configurations. Nonetheless, the invention will be
described and explained with additional specificity and detail
through the use of the accompanying drawings in which:
[0011] FIG. 1 is a schematic illustration of a slow solvent
evaporation system used in accordance with one embodiment of the
present invention.
[0012] FIG. 2 shows (A) SEM image of a nanofibril film deposited on
a glass slide. (B) Zoom-in SEM image of the nanofibril film. (C, D)
Bright-field and fluorescence optical microscopy image of a
nanofibril film. Note that due to the diffraction effect the fiber
in the optical microscopy image appears larger than the real size
as measured by SEM.
[0013] FIG. 3A is a fluorescence quenching efficiency (1-I/I.sub.0)
as a function of the vapor pressure of aniline: data (error 5%)
fitted with the Langmuir equation.
[0014] FIG. 3B is a fluorescence spectra of a nanofibril film
before (red) and after (blue) exposure to the saturated vapor of
aniline (880 ppm) for 10 s.
[0015] FIG. 3C shows the absorption (black) and fluorescence (red)
spectra of molecule 1 in chloroform solution (dashed) and the
nanofiber state (solid). The raised baseline for the absorption
spectrum of nanofibril film is primarily due to the light
scattering.
[0016] FIG. 4 shows energy levels of HOMO (.pi.) and LUMO (.pi.*)
orbitals of 1 and aniline showing the favorable electron transfer
from amine to the photoexcited state of 1. The same diagram applies
to the other amines, while the reducing power (or the .pi.-orbital
level) would be different from that of aniline (see Table 1).
Geometry optimization and energy calculation were performed with
density-functional theory (B3LYP/6-311 g**// B3LYP/6-31 g*) using
Gaussian 03 package.
[0017] FIG. 5 is a comparison between the fluorescence spectra of
the nanofibril film of 1 (black) and a thin film (red) drop-cast
from the THF solutions of a PTCDI molecule modified by two bulky,
branched side-chains,
N,N'-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide
(HH-PTCDI), which forms ill-shaped aggregates, mainly due to the
significant steric hindrance caused by the large side-chains.
[0018] FIG. 6 is a time-course of fluorescence quenching of a
nanofibril film upon blowing over with saturated vapor of aniline
(880 ppm), indicating a response time of about 0.32 s. The
intensity was monitored at 628 nm.
[0019] FIG. 7A is a fluorescence spectra of a nanofibril film
before (black) and after (red) exposure to diluted vapor of
aniline
[0020] FIG. 7B is a fluorescence spectra of a nanofibril film
before (black) and after (red) exposure to diluted vapor of
hydrazine.
[0021] FIG. 8 is a fluorescence spectra of a nanofibril film after
continuous irradiation at 550 nm for 0, 10, 20, 30, 60 min. The
film was held in the LS55 fluorometer with a constant excitation
slit of 5 nm and a pulsed Xenon discharge lamp (7.3 W) as the light
source. The unchanged fluorescence indicates the robust
photostability of the film.
[0022] FIG. 9 is a bar graph of fluorescence response of the
nanofibril film to various organic reagents: 1, methanol; 2,
acetone; 3, acetic acid; 4, THF; 5, acetonitrile; 6, chloroform; 7,
toluene; 8, hexane; 9, cyclohexane; 10, nitromethane; 11,
nitrobenzene; 12, phenol; 13, cyclohexylamine; 14, dibutylamine;
15, aniline; 16, butylamine (3 s); 17, triethylamine; 18,
hydrazine; 19, ammonium hydroxide. Unless otherwise marked, the
exposure times for the amines and all the other reagents are 10 and
15 s, respectively.
[0023] FIG. 10 shows five continuous cycles of quenching-recovery
which were tested for a nanofibril film upon exposure to the
saturated vapor of phenol (335 ppm). The quenching was performed by
exposing the film to the phenol vapor for 15 s. After each cycle of
quenching, the fluorescence of the film was recovered by exposing
the film to an open air for 60 min or at an elevated temperature
(60.degree. C.) for 5 min.
[0024] FIG. 11 (A) SEM image of the nanofibers deposited on a glass
slide. (B) Fluorescence optical microscopy image of a nanofibril
film deposited on a glass slide.
[0025] FIG. 12 (A) Fluorescence spectra of a nanofibril film after
60 s of exposure to aniline vapor at 35, 70, 175, 350, 525, 875,
1750 ppb. (B) Fluorescence spectra of a nanofibril film measured 15
min (red) and 30 min (blue) after the complete quenching (shown in
black) in the presence of 1750 ppb aniline vapor. No spectral
change with time indicates irreversibility of the quenching
process. The same test was also performed with the larger fibers
(350 nm in diameter) as shown in the inset, where significant
recovery of the fluorescence emission was observed.
[0026] FIG. 13 (A) Fluorescence quenching efficiency (1-I/I.sub.0)
as a function of the vapor concentration aniline, measured for the
nanofibril films deposited from both the ultrathin nanofibers
(30-50 nm) and large fibers (350 nm); comparative investigation was
performed on three different films fabricated from the ultrathin
nanofibers using various amount of fibril materials (cyan: 0.35 mg;
blue: 0.15 mg; red: 0.1 mg) as well as a film fabricated from the
large fibers (black: 0.35 mg). (B) Fitting the three sets of data
in (A) with the Langmuir equation aiming to predict the detection
limit based on the common photon detection threshold of PMT. All
data are with an error of .+-.3%.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0027] The following detailed description of exemplary embodiments
of the invention makes reference to the accompanying drawings,
which form a part hereof and in which are shown, by way of
illustration, exemplary embodiments in which the invention may be
practiced. While these exemplary embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention, it should be understood that other embodiments may
be realized and that various changes to the invention may be made
without departing from the spirit and scope of the present
invention. Thus, the following more detailed description of the
embodiments of the present invention is not intended to limit the
scope of the invention, as claimed, but is presented for purposes
of illustration only and not limitation to describe the features
and characteristics of the present invention, to set forth the best
mode of operation of the invention, and to sufficiently enable one
skilled in the art to practice the invention. Accordingly, the
scope of the present invention is to be defined solely by the
appended claims.
[0028] Definitions
[0029] In describing and claiming the present invention, the
following terminology will be used.
[0030] The singular forms "a," "an," and "the" include plural
referents unless the context clearly dictates otherwise. Thus, for
example, reference to "a binding moiety" includes reference to one
or more of such groups and reference to "exposing" refers to one or
more such steps.
[0031] As used herein, "alkylene" refers to a saturated hydrocarbon
having two valencies, i.e. for bonding with adjacent groups.
Non-limiting examples of alkylenes include --CH--, --CH.sub.2--,
--C.sub.2H.sub.4--, --C.sub.3H.sub.6--, etc. This is in contrast to
"alkyl" groups which are similar but have a single valency and
include at least one CH.sub.3 end group.
[0032] As used herein, when referring to a component of a
composition, "primarily" indicates that that component is present
in a greater amount than any other component of the relevant
composition.
[0033] As used herein with respect to an identified property or
circumstance, "substantially" refers to a degree of deviation that
is sufficiently small so as to not measurably detract from the
identified property or circumstance. The exact degree of deviation
allowable may in some cases depend on the specific context.
[0034] As used herein, a plurality of items, structural elements,
compositional elements, and/or materials may be presented in a
common list for convenience. However, these lists should be
construed as though each member of the list is individually
identified as a separate and unique member. Thus, no individual
member of such list should be construed as a de facto equivalent of
any other member of the same list solely based on their
presentation in a common group without indications to the
contrary.
[0035] Concentrations, amounts, and other numerical data may be
presented herein in a range format. It is to be understood that
such range format is used merely for convenience and brevity and
should be interpreted flexibly to include not only the numerical
values explicitly recited as the limits of the range, but also to
include all the individual numerical values or sub-ranges
encompassed within that range as if each numerical value and
sub-range is explicitly recited. For example, a numerical range of
about 1 to about 4.5 should be interpreted to include not only the
explicitly recited limits of 1 to about 4.5, but also to include
individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3,
2 to 4, etc. The same principle applies to ranges reciting only one
numerical value, such as "less than about 4.5," which should be
interpreted to include all of the above-recited values and ranges.
Further, such an interpretation should apply regardless of the
breadth of the range or the characteristic being described.
[0036] Any steps recited in any method or process claims may be
executed in any order and are not limited to the order presented in
the claims unless clearly indicated otherwise. Means-plus-function
or step-plus-function limitations will only be employed where for a
specific claim limitation all of the following conditions are
present in that limitation: a) "means for" or "step for" is
expressly recited; and b) a corresponding function is expressly
recited. The structure, material or acts that support the
means-plus function are expressly recited in the description
herein. Accordingly, the scope of the invention should be
determined solely by the appended claims and their legal
equivalents, rather than by the descriptions and examples given
herein.
[0037] Fluorescent Sensor Compounds for Detecting Amines
[0038] A new type of fluorescence sensor for expedient vapor
detection of organic amines with both high sensitivity and
selectivity is provided. The sensing mechanism is primarily based
on quenching of the fluorescence emission of the sensory materials
upon interaction with the amine molecules. The sensory materials
can be composed of well-defined nanofibers fabricated from an
n-type organic semiconductor molecule. The long-range exciton
migration intrinsic to the one-dimensional well-organized molecular
arrangement within the nanofiber enables amplified fluorescence
quenching by the surface adsorbed analytes (quencher molecules).
Upon deposition onto a substrate, the entangled nanofibers form a
mesh-like, highly porous film, which provides maximal adsorption
and accumulation of the gaseous molecules under detection, leading
to expedient vapor sensing of amines with unprecedented efficiency
(down to detection limit in ppt range).
[0039] Fluorescent sensor compounds for detecting amines can be
3,4,9,10-tetracarboxyl perylene compounds can generally have the
formula I:
##STR00003##
[0040] where A and A' are independently chosen from N--R1, N--R2,
and O such that both A and A' are not O, and R1 through R10 are
amine binding moieties, solubility enhancing groups, or hydrogen
such that at least one of R1 through R10 is an amine binding
moiety. Typically, the fluorescent sensor compounds can be formed
into a nanofiber structure although this is not required.
[0041] In one specific aspect, the fluorescent sensor compound can
be an imide-anhydride perylene where A is N--R1 and A' is O.
Formula II illustrates one specific class of imide-anhydride
perylenes where R3-R6 and R7-R10 are hydrogen.
##STR00004##
[0042] In this case, the anhydride moiety (O.dbd.C--O--C.dbd.O) is
an amine binding moiety which does not have steric hindrance
sufficient to disrupt formation of one-dimensional self-assembly of
the compound into a nanofibril structure of the present invention.
The group R1 can be chosen to provide solubility of the compound in
the organic solvent and which also does not disrupt self-assembly
into a nanofibril structure. Such disruption may not be undesirable
if nanofibrils are not the intended final product morphology. In
one aspect, R1 is a C1 to C13 alkyl chains which can be straight or
branched. Non-limiting examples of branched alkyls for R1 can
include symmetric branched alkyls such as hexylheptyl, pentylhexyl,
and butylpentyl. However, asymmetric branched alkyls can also be
suitable such as butylheptyl, 4-methyl-1-hexylheptyl, and the like.
As a general rule smaller alkyl chains such as methyethyl and
propylbutyl tend to exhibit low solubility, depending on the
particular molecule.
[0043] In another alternative, the 3,4,9,10-tetracarboxyl perylene
compound can be a bisimide, i.e. A is N--R1 and A' is N--R2. Each
of R1 and R2 can be C1 to C13 alkyl groups as discussed above.
Furthermore, carboxylic acid can be a side group which is added to
act as the amine binding group. Formula III illustrates one
alternative class of carboxylic acid bisimides of the present
invention.
##STR00005##
[0044] The solubility enhancing groups can be oriented as side
groups (R3-R10) or as in Formula III at R1 to control or increase
solubility of the compound during manufacture of nanofibers
Although other solubility enhancing groups (R1) can be suitable as
outlined herein, one embodiment of formula III can include
symmetric alkyl groups such as, but not limited to, hexylheptyl,
pentylhexyl, and butylpentyl.
[0045] In still another alternative embodiment, the
3,4,9,10-tetracarboxyl perylene compound can include carboxylic
acid and/or anhydride moieties. Such side groups can be useful to
provide amine binding groups. As discussed in more detailed below,
such solution processing usually involves self-assembly mechanisms.
In some embodiments, the solubility enhancing moieties can be
located along the sides of the perylene core, i.e. R3-R6 and
R7-R10. However, most often the amine binding moieties can be
located along sides of the perylene core. Formulas IV-VI illustrate
several carboxylic acid and anhydride substituted perylene
compounds suitable for use in the sensor compounds.
##STR00006##
[0046] Formula IV illustrates a compound having a maleic anhydride
moiety formed collectively of R4 and R5. Formula V and IV
illustrate 3,4,9,10-tetracarboxyl perylene compounds which include
carboxylic acids as side groups, although almost any combination or
number of R3 through R10 can be COOH, one or two carboxylic acid
groups are most typical. The R1 and R2 end groups can be chosen
from among those previously listed. However, C5-C12 cycloalkyls can
also be employed as the side-chains substituted at the imide
position (A or A') as the solubility enhancing groups to facilitate
solution processing. These cycloalkyl groups are suitable for
one-dimensional self-assembly of the molecules into nanofibrils.
For example, non-limiting examples of cycloalkyls can include
cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, and cyclododecyl.
Specific examples of amine binding sites can include an oxygen
moiety like anhydride or an acid like --COOH. While many of the
amine-binding moieties mentioned above can be substituted at the
bay area, some of the bulky alkyl groups like the branched ones are
generally not suitable for substitution at the bay area when
forming nanofibers, since they will distort the pi-pi stacking
between the perylene planes mainly due to the increased steric
hindrance.
[0047] The nanofiber-based fluorescent sensor compounds can be
formed via synthesis of the underlying perylene compound which is
then formed into the nanofibers. For example, a
3,4,9,10-tetracarboxyl perylene compound having the Formula I (as
previously noted) can be synthesized. In one specific example, the
starting compound used for synthesizing the sensor molecule,
3,4,9,10-tetracarboxylic perylene dianhydride (Formula I with A and
A' both as O, and R3-R10 as hydrogen) can be obtained commercially
from many chemical manufacturers including Sigma Aldrich and Fisher
Scientific. A diimide compound synthesized from the dianhydride can
be subjected to partial hydrolysis to form an anhydride imide such
as those described by Formula II above.
[0048] The perylene compound can be self-assembled into nanofibers
via any suitable process such as, but not limited to, a slow
controlled solvent-exchange step, rapid solution dispersion, phase
transfer at the interface between two solvents, sol-gel processing,
direct vaporization of the solvent, or any other suitable
self-assembly methods including the surface assisted process. A
more detailed description of some of these options can be found in
a recent publication, Ling Zang, Accounts of Chemical Research, a
special issue in Nanoscience, 41 (2008) 1596-1608, which is
incorporated herein by reference. The slow controlled
solvent-exchange step can be accomplished by dissolving the
compound in a suitable good solvent, e.g. dichloromethane,
chloroform, tetrachloromethane, alkanes, etc., which typically have
a solubility of at least 0.2 mM and in some cases at least 1 mM
concentration for the perylene compound. A solution of the perylene
compound can be placed in a closed chamber in proximity to a poor
solvent (e.g. some solubility for the perylene compound but
generally less than about 1 .mu.M concentration and in some cases
less than about 0.01 mM). Poor solvents can vary depending on the
particular perylene compound but can often include methanol,
ethanol, hexane, heptane, cyclohexane, acetonitrile, etc. Vapor
diffusion between the two solvents will gradually decrease the
concentration of good solvent in the perylene solution and the
solubility of the solution. As a result the perylene compound
begins to crystallize slowly into the nanofibers of the present
invention. The rate of nanofiber formation can depend on the
particular solvents, temperature, etc., but is often about a day to
reach equilibrium. The ultrathin nanofibers (20-50 nm in diameter)
can be fabricated via a quick crystallization method, e.g.
injecting the good solvent solution of perylene monoimide (e.g. 0.3
mL, 3.4 mM) into poor solvent (e.g. hexane, 1.2 mL) in a small test
tube, followed by 30 min aging.
[0049] The nanofibers can vary in size, depending on the specific
perylene compound used. However, as a general guideline, the
nanofibers can have a diameter from about 10 nm to about 1000 nm,
in some cases to about 500 nm, and one aspect from about 100 nm to
about 350 nm while in another aspect from about 10 nm to about 50
nm. Similarly, the length of the nanofibers can vary considerably
but is often from about 1 .mu.m to about 1 mm, and in some cases
from about 10 .mu.m.
[0050] The formed nanofibers can then be suspended in a liquid
vehicle in which the nanofibers are very poorly soluble, e.g. less
than about 1 micromolar concentration, at least less than 0.01 mM,
or completely insoluble, to form a nanofiber dispersion.
Non-limiting examples of suitable liquid vehicles can include
hexane, heptane, methanol, cyclohexane, alcohols, and the like.
[0051] The nanofiber fluorescent sensor compound can be formed into
a film of entangled nanofibers by coating the nanofiber dispersion
on a substrate and allowing the solvent to evaporate.
[0052] The formed nanofibers have shown rapid fluorescence
responses upon exposure to various amine compounds. The nanofiber
fluorescent sensor compound can be exposed to a fluid sample in
which the nanofiber fluorescent sensor compound is not
substantially soluble. The fluid sample can generally be a fluid
containing the target gaseous analyte, although liquids can also be
tested. A fluorescence change can be measured and/or displayed upon
exposure of the nanofiber sensor compound to the fluid sample.
Typically, the fluorescence change can be accomplished using a
fluorometer, or simply a photon detector that can measure the
fluorescence emission intensity. Depending on the application, the
displaying of fluorescence change can be a quantitative measure of
fluorescence response, e.g. a percentage change of luminescence
intensity. Alternatively, the displaying is qualitative such as by
visual observation of a fluorescence change. Such qualitative
measure can be sufficient when the mere presence of a particular
amine is sought rather than an absolute measure of the
concentration.
[0053] The specific performance of individual perylene nanofibers
can vary. However, in one aspect of the invention, the nanofiber
fluorescent sensor compound can exhibit a fluorescence change (e.g.
quenching) from 50% to 100% for a majority of amines selected from
the group consisting of phenol, cyclohexylamine, dibutylamine,
aniline, butylamine, triethylamine, hydrazine, and ammonium
hydroxide. Furthermore, the fluorescence change for each of
cyclohexylamine, dibutylamine, aniline, butylamine, triethylamine,
hydrazine, and ammonium hydroxide can most often be from about 80%
to about 100%.
[0054] Advantageously, the nanofiber fluorescent sensor compounds
can be regenerated by dissolving the nanofiber fluorescent sensor
compound and regenerating the nanofibers as previously described.
It is noted that such regeneration also does not typically involve
a chemical reaction, but rather dissolving of the perylene compound
in a suitable solvent and repeating the self-assembly process
previously described. Thus, although not generally regenerable by
an end user, the sensor compound can be readily collected and
recycled with no residual effects on the performance of the
material.
[0055] Furthermore, the fluorescent sensor compounds can be used as
fluorescent dyes or other applications such as in solar cells and
the like which do not require nanofiber morphology. This technology
can also find a broad range of applications in health and security
examination, where instant detection of trace amine is usually
demanded. Indeed, sensitive vapor detection of organic amines is
not only critical to the air pollution monitoring and control, but
will also provide expedient ways for food quality control, and even
medical diagnosis of certain types of disease, e.g., uremia and
lung cancer, for which biogenic amines released are usually used as
the biomarkers.
[0056] Compared to the electrical sensors like those based on
chemiresistors, the reported fluorescent (optical) sensor system
represents a class of simple, expedient technique for chemical
vapor detection and analysis. In contrast to the polymer film-based
fluorescent sensors, the nanofibril film-based sensors provide
three-dimensional continuous pores (or channels) formed by the
entangled piling of the nanofibers, enabling expedient diffusion of
the analyte molecules throughout the film matrix, and thus fast
response (milliseconds) for the sensing. The high porosity (and
thus large surface area) formed by the entangled piling of
nanofibers also provides maximal adsorption and accumulation of the
gaseous molecules under detection, leading to expedient vapor
sensing of amines with unprecedented efficiency (down to detection
limit in ppt range). The nanofibril materials, as well as the new
sensing module thus developed, can open wide options to improve the
detection efficacy and find broad range of applications in health
and security examination, where instant detection of trace amine is
usually demanded.
[0057] Perylene-tetracarboxylic diimide (PTCDI) represents a robust
class of n-type organic materials with strong photostability, which
is particularly desirable for being used in optical sensing or
probing regarding both the performance sustainability and
reproducibility. The sensor compounds can find broad applications
in health and security examination. For example, air quality and
security industries can benefit from real time amine detection.
In-field monitoring of air quality against pollution by toxic
amines is one example, which have commonly been used in various
industry and military systems. Particularly, hydrazine has been
heavily used in both industry (as an oxygen scavenger and corrosion
inhibitor) and military (as a fuel in rocket propulsion systems).
Moreover, this compound has been implicated as a carcinogen and is
readily absorbed through the skin. Another typical toxic amine is
ethanolamine, which has been used as the scrubbing agent in the
ventilation system of submarines to remove carbon dioxide from the
air. Due to their toxicity and reactivity, facile detection of
these amines is relevant to both life and environment security.
[0058] Health and clinic applications can include rapid screening
of uremia and lung cancer, one of the most common cancers,
particularly in the developing countries. Alkyl-amines will be used
as the biomarkers for uremia diseases, while aromatic-amines will
be used for lung cancer. Very trace amount of amines breathed out
of the patient will be detected (at concentration of ppt), thus
enabling rapid diagnostics or warning of the diseases at the early
stage. Food industry applications can include high throughput
quality control and monitoring by detecting the amines released
from foods.
[0059] A new type of fluorescence sensory material with high
sensitivity, selectivity, and photostability has been developed for
vapor detection of organic amines. The sensory material is
primarily based on well-defined nanofibers fabricated from an
n-type organic semiconductor molecule. Upon deposition onto a
substrate, these entangled nanofibers form a meshlike, highly
porous film, which allows for maximal exposure to the gaseous
analyte molecules, expedient diffusion of the molecules throughout
the meshlike film, and increased adsorption and accumulation of the
gaseous molecules within the porous matrix.
[0060] Compared to the electrical sensors like those based on
chemiresistors, the reported fluorescent sensor system represents a
class of simple, expedient technique for chemical detection and
analysis. In contrast to the polymer-film-based fluorescent
sensors, the nanofibril-film-based sensors provide
three-dimensional continuous pores (or channels) formed by the
entangled piling of the nanofibers, enabling expedient diffusion of
the analyte molecules throughout the film matrix, and thus fast
response (milliseconds) for the sensing. The nanofibril materials,
as well as the new sensing module thus developed, may open wide
options to improve the detection efficacy and find broad range of
applications in health and security examination, where instant
detection of trace amine is highly beneficial.
EXAMPLE 1
[0061] A strongly fluorescent n-type organic semiconductor
material, which can be fabricated into well-defined nanofibers and
employed in efficient fluorescent probing of gaseous amines is
described. Without being bound to any particular theory, it is
thought that the long-range exciton migration intrinsic to the
one-dimensional well-organized molecular arrangement within the
nanofiber enables amplified fluorescence quenching by the surface
adsorbed analytes (quencher molecules). Taking advantage of such
amplified fluorescence quenching intrinsic to nanofibers, a new
type of nanofibers was fabricated from an n-type material that can
be used for effective sensing of reductive compounds, such as
organic amines, through electron-transfer-based fluorescence
quenching. The building block molecule (1) employed for the
nanofibril fabrication is shown in Formula V, which was synthesized
through partial hydrolysis of hexylheptyl substituted
3,4,9,10-perylene-tetracarboxylic diimide (PTCDI).
##STR00007##
[0062] In particular,
N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide
(1) was synthesized by suspending 1 g
N,N'-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide (1.3
mmol) in 60 mL of t-BuOH containing 700 mg solid KOH (85%). The
mixture was heated with vigorous stirring to reflux. After
refluxing for 1.5 h, the reaction solution was cooled to room
temperature, followed by addition of 50 mL of 2 M HCl, followed by
stirring over night. The resulting solid was collected by vacuum
filtration through a 0.45 .mu.m membrane filter (Osmonics). The
solid was then washed thoroughly with distilled water until the pH
of washings turned to be neutral. The hydrolyzed product from
N,N'-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide was
directly purified by column chromatography (eluent: methylene
chloride), yielding 0.35 g (35%) of molecule 1, having the
following properties: .sup.1H-NMIR (CDCl.sub.3): .delta.=0.83 (t,
6H, 2CH.sub.3), 1.17-1.42 (m, 16H, 8CH.sub.2), 1.85 (m, 2H,
CH.sub.2), 2.24 (m, 2H, CH.sub.2), 5.19 (m, 1H, CH), 8.67 (m, 8H,
perylene).
[0063] Self-assembly of molecule 1 into nanofibers was performed
through a slow solvent-exchange process, which was realized via
vapor diffusion within a closed chamber. Briefly, a test tube
containing about 0.2 mL CHCl.sub.3 solution of 1 (1.7 mM) was
placed in a 50 mL jar, which contained about 10 mL of methanol,
followed by sealing the jar for slow vapor diffusion between the
two solvents (FIG. 1). Upon gradual solvent exchange, the solution
in the test tube became more dominant with methanol, which is a
poor solvent (with low solubility) for molecule 1, thereby leading
to self-assembly of the molecules into nano fibers.
[0064] Because of the slow crystallization process controlled by
the slow vapor diffusion, the nanofibers fabricated via such a
process are usually in a well-defined shape and sizes as shown in
FIG. 2A-D. After about one day the exchange between the two
solvents reached the equilibrium, resulting in complete assembly of
the molecules, and precipitating down to the bottom of the test
tube. The nanofibers thus obtained were re-dispersed in hexane,
producing a suspension well-suited for deposition on a substrate
either for microscopy imaging or vapor sensing tests. For each of
the sensing tests, the whole amount of the nanofibers thus prepared
were deposited on a glass substrate to produce a film that
maintained the same surface area (adsorption) for all the sensing
tests as presented in FIG. 3A.
[0065] The fluorescence quantum yield (.PHI.) of the nanofibril
film was estimated by measuring the absorption and fluorescence
intensity in comparison with a thin-film fluorescence standard with
.PHI.=100%. The thin-film standard was prepared by sandwiching one
drop of a polystyrene/toluene gel between two glass cover slips.
Within the gel was dissolved an appropriate concentration of a
PTCDI molecule,
N,N'-di(hexylheptyl)-perylene-3,4,9,10-tetracarboxyl-diimide
(HH-PTCDI). By maintaining molecular dispersion of the molecules
within the gel, the fluorescent quantum yield of HH-PTCDI remains
100%, as it is dissolved in a homogeneous solution in toluene or
other good organic solvents.
[0066] Molecule 1 possesses a structure that provides a good
balance between the molecular stacking and the fluorescence yield
of the materials thus assembled. The former prefers a molecular
structure with minimal steric hindrance (usually referring to a
small or linear side chain), while the latter favors bulky,
branched side chains that may distort the .pi.-.pi. stacking to
afford increased fluorescence (by enhancing the low-energy
excitonic transition) for the molecular assembly. FIG. 2A shows the
scanning electron microscopy (SEM) image of the nanofibers
fabricated from molecule 1 through the vapor-diffusion (slow
solvent exchange) process as described in FIG. 1. The average
diameter of the nanofibers was .about.350 nm as determined by
zoom-in SEM imaging as shown in FIG. 2B.
[0067] The extended one-dimensional molecular arrangement obtained
for molecule 1 is likely dominated by the .pi.-.pi. interaction
between the perylene backbones (which is sterically favored by the
bare end of molecule 1), in cooperation with the hydrophobic
interactions between the side chains in appropriate size. Such a
molecular arrangement is reminiscent of the one-dimensional
self-assembly commonly observed for detergents, lipids, or
amphiphilic peptides, for which extended molecular assembly can be
achieved through the concerted electrostatic and hydrophobic
interactions. It seems that one-dimensional molecular assembly of
molecule 1 is dependent on the size of the side chains. Replacing
the side chain of molecule 1 with a larger group, for example,
nonyldecyl, resulted in formation of only ill-shaped molecular
aggregates. The nanofibers fabricated from molecule 1 demonstrates
strong fluorescence with yield .about.15% as depicted in the
fluorescence microscopy images (FIG. 2C and FIG. 2D), implying a
distorted molecular stacking that is usually observed for the PTCDI
molecules modified with branched side chains. The strong red
fluorescence of the nanofibers can easily be observed even with the
naked eye, making the nanofibers more feasible to be used in
qualitative fluorescence sensing. Although not essential to an
understanding of the compounds, an illustrative movie clip shows a
demonstration of one embodiment of this compound and can be found
at pubs.acs.org (Supporting Information for Nano Lett., 2008, 8(8),
2219-2223, which article is incorporated herein by reference) or
www.chem.siu.edu/zang/image/gas-sensor.wmv.
[0068] FIG. 3C shows the absorption and fluorescence spectra
measured from the nanofibers deposited on glass substrate, in
comparison to the spectra measured for molecule 1 dissolved in a
chloroform solution. The electronic property of molecule 1 as
depicted in FIG. 3C is quite similar to the parent PTCDI molecules
with the HOMO-LUMO gap around 2.5 eV, consistent with the ab initio
calculation results (FIG. 4).
TABLE-US-00001 TABLE 1 Physical properties and quenching results of
various amines and phenol Oxidation Vapor Quenching potential.sup.a
Driving pressure.sup.b efficiency E.sub.1/2 value(V) force.sup.a
ppm at (10 s of Analyte vs SCE .DELTA. G (-eV) 25.degree. C.
exposure) (%) Butylamine 1.52 0.62 120400 96 Pentylamine 1.69.sup.c
0.45 39480 95 Hexylamine .sup. 1.72.sup.d 0.42 8580 95 Octylamine
-- -- 1280 94 Dibutylamine 1.20 0.94 3360 91 Triethylamine 0.99
1.15 75990 85 Cyclohexylamine .sup. 1.72.sup.d 0.42 11840 94
Cyclopentylamine -- -- -- 94 Aniline 0.86 1.28 880 95 Hydrazine
0.43 1.71 5920 98 Phenol 1.37 0.77 340 54 .sup.aThe driving force
for the fluorescence quenching, i.e., photoinduced electron
transfer from the analyte to 1 was calculated using the Rehm-Weller
equation: .DELTA.G = -e(E.sup.o.sub.red - E.sup.o.sub.ox) -
.DELTA.E.sub.oo, where E.sup.o.sub.red and E.sup.o.sub.ox are the
reduction potential of electron acceptor and the oxidation
potential of electron donor, respectively, and .DELTA.E.sub.oo is
the singlet excitation energy. .sup.bThe vapor pressure data are
cited from CRC handbook of Chemistry and Physics, 85th Edition, CRC
Press, 2004, p15-16 to 25. .sup.cThe oxidation potential of
pentylamine (determined as the peak potential). .sup.dThe oxidation
potentials of hexylamine and cyclohexylamine. The relatively lower
quenching efficiency observed for the tertiary amines might be due
to the weaker chemical binding with the anhydride, with which the
binding of a tertiary amine is primarily through the donor-acceptor
interaction, but lack of hydrogen bonding.
[0069] The fluorescence quantum yield of molecule 1 in solution is
.about.100%, the same as other PTCDI molecules tested. Upon
assembly into the solid state, the fluorescence of individual
molecules disappeared, while a new emission band formed at a longer
wavelength centered around 628 nm. Compared to the emission
spectrum (0.21 eV fwhm) obtained from the ill-shaped molecular
aggregate formed from the parent PTCDI molecule modified with two
hexylheptyl side chains (FIG. 5), the emission measured for the
nanofibers of molecule 1 exhibits a significantly narrower band,
only 0.17 eV fwhm, implying the well-organized molecular assembly
within the nanofibers. Consistently, a new, pronounced band was
observed at the longer wavelength in the absorption spectrum of the
nanofibers, which is typically characteristic of the strong
.pi.-.pi. interaction as observed in the self-assemblies of PTCDI
and other planar .pi.-conjugated molecules. The strong .pi.-.pi.
interaction is also revealed by the characteristic enhancement of
the transitions (absorptions) from ground state to the higher
levels of electronic states (0-1, 0-2, and 0-3, compared to 0-0) of
the component molecules. The strong .pi.-.pi. interaction may
enhance the exciton migration, which is now more confined along the
long axis of the nanofiber, leading to amplification in
fluorescence quenching by the surface adsorbed analytes
(quenchers).
[0070] Upon fabrication from hydrophilic solvents such as alcohols,
the nanofibers are expected to possess a surface predominantly
consisting of the anhydride moieties, which are more hydrophilic
compared to the hexylheptyl group located at the other end of the
molecule. A surface full of anhydride moieties enables strong
chemical binding or adsorption with amines through both hydrogen
bonding and donor-acceptor (charge transfer) interaction.
Deposition of the nanofibers onto a substrate produces a meshlike
film that is primarily formed by entangled piling of the fibers and
thus possesses porosity on a number of length scales (FIG. 2A-D).
Such a porous film not only provides increased surface area for
enhanced adsorption of gaseous molecules but also enables expedient
diffusion of guest molecules across the film matrix, leading to
efficient probing of the gaseous molecules with both high
sensitivity and fast time response.
[0071] The organic compounds employed for sensing tests include
methanol, acetone, acetic acid, THF, acetonitrile, chloroform,
toluene, hexane, cyclohexane, nitrobenzene, nitromethane, phenol,
cyclohexylamine, bibutylamine, aniline, butylamine, triethylamine,
hydrazine, and ammonium hydroxide. All the compounds and/or
solvents (HPLC or spectroscopic grade) were purchased from Fisher
or Aldrich, and used as received.
[0072] UV-vis absorption and fluorescence spectra were measured on
a PerkinElmer Lambda 25 spectrophotometer and LS 55 fluorometer,
respectively. SEM measurement was performed with a Hitachi S570
microscope (operated at 10 kV). The sample was prepared by casting
one drop of the nanofiber suspension in hexane onto a clean glass
cover slip, followed by drying in air and then annealing overnight
in an oven at 45.degree. C. The dried sample was coated with gold
prior to the SEM imaging. The bright-field optical and fluorescence
microscopy imaging was carried out with a Leica DMI4000B inverted
microscope, using a Rhodamine filter set, which provides excitation
in the range of 530-560 nm, and collects emission at >580
nm.
[0073] The fluorescence quenching by amines vapor was monitored.
Briefly, the fluorescence spectra of the film were measured
immediately after immersing inside a sealed-jar (50 mL) containing
small amount of the amines. To prevent direct contact of the film
with the amines, some cotton was placed above the amines (deposited
at the bottom of the jar). Before use the jar was sealed for
overnight to achieve saturated vapor inside. The presence of cotton
also helps maintain a constant vapor pressure. The fluorescence
quenching at the diluted vapor pressures of amines (e.g., aniline
and hydrazine) was performed in a sealed cuvette (5 mL volume),
into which a small volume of the saturated vapor of a specific
amine was injected (using an air-tight micro-syringe) to achieve
the diluted vapor. For example, injection of 5 .mu.L of saturated
aniline vapor (880 ppm) into the 5 mL cuvette will produce a vapor
pressure 1000 times diluted, e.g., 880 ppb. The lowest vapor
pressure of aniline that can be achieved through vapor dilution was
about 35 ppb, for which two steps of dilution were carried out,
i.e., 50 .mu.L of the ambient saturated vapor of aniline was
injected into a 5 mL jar immersed in a water bath (ca. 70.degree.
C., to avoid minimal condensation of the vapor), followed by
injecting 20 .mu.L of this diluted vapor into the 5 mL cuvette.
[0074] The time-dependent fluorescence quenching profile (shown in
FIG. 6) was measured with an Ocean Optics USB4000 fluorometer,
which can be switched to the mode to measure the emission intensity
at a selected wavelength as a function of time. An open sample
holder (Ocean Optics, CUV-ALL-UV) was used to hold the nanofibril
film deposited on a glass cover slip, and the fluorescence from the
nanofibers was collected at 90.degree. with respect to the
excitation beam, which was provided by an Ar.sup.+ laser (Melles
Griot) tuned at 488 nm. Both the excitation and emission were
transported with 0.6 mm premium UV/Vis fibers (Ocean Optics). The
fluorescence quenching was carried out by blowing a few mL of
saturated aniline vapor (880 ppm) onto the nanofibril film during
the course when the emission was continuously recorded by the
fluorometer.
[0075] Indeed, as shown in FIG. 6, upon exposure to the saturated
vapor of aniline (880 ppm) the fluorescence of the nanofibril film
was instantaneously quenched by almost 100%. Such efficient
fluorescent sensing was also observed for a broad range of amines
(primary, secondary, and tertiary) as listed in Table 1 above. The
fluorescence quenching thus observed is due to a photoinduced
electron transfer process as depicted in FIG. 4 where the electron
transfer is driven by the favorable energy difference between the
HOMO of aniline and the HOMO of PTCDI (which is now one electron
vacant in the excited state). The high efficiency of the
fluorescence quenching is consistent with the large driving force
for the photoinduced electron transfer between the excited state of
molecule 1 and the amine molecules (FIG. 4 and Table 1).
[0076] To explore the detection limit for some of the
representative amines such as aniline and hydrazine, the same
quenching process shown in FIG. 3B was also examined for the
diluted amine vapor. FIG. 3A shows the fluorescence quenching
efficiency (1-I/I.sub.0) of a nanofibril film measured at four
different vapor pressures of aniline, 1, 1000, 10,000 and 25,000
times diluted from the saturated vapor (880 ppm) at room
temperature. The quenching data are well fitted to the Langmuir
equation with an assumption that the quenching efficiency is
proportional to the surface adsorption (coverage) of amines. From
the fitted plot the detection limit of the nanofibril film shown in
FIG. 3A can be projected as low as .about.200 ppt, if considering
the fact that a well-calibrated photodetector (e.g., PMT) can
detect intensity change as small as 0.1% or below. Following the
same procedure the detection limit for hydrazine was estimated to
be .about.1 ppb.
[0077] FIG. 6 shows the emission intensity of the film monitored as
a function of the time after exposed to the saturated vapor of
aniline (880 ppm). Fitting the intensity decay into a single
exponential kinetics deduces a response time for the quenching
process (defined as the decay lifetime), only 0.32 s. The fast
response thus obtained for the nanofibril sensor is mainly due to
the three-dimensional continuous, porous structure formed by the
entangled piling of the nanofibers, which allows for expedient
diffusion of the analyte molecules throughout the film matrix, thus
leading to instant capture (and accumulation) of the vapor species.
The fast sensing response, along with the low detection limits and
the robust photostability (zero photobleaching, as shown in FIG. 8)
observed, makes the nanofibril film an ideal probing system in a
broad range of applications, particularly for onsite amine
monitoring and screening, where instant vapor detection of trace
amines is usually demanded.
[0078] The nanofibril film also demonstrated high selectivity in
response to organic amines, with minimal fluorescence quenching
observed for other common organic reagents, such as those listed in
FIG. 9. For all the amines tested, more than 85% fluorescence
quenching was observed for the nanofibril film upon exposure to the
saturated vapor of amines, whereas all the other organic liquids
and solids (except for phenol) examined as the potential background
interference exhibited less than 3% fluorescence quenching under
the same testing conditions (FIG. 9). The significant quenching
(.about.54%) observed with phenol is likely due to its strong
reducing power, i.e., electron-donating capability.
[0079] Interestingly, the fluorescence quenching observed with
phenol was highly reversible as shown in FIG. 10 (without chemical
reaction), where the fluorescence of the nanofibril film after
exposure to the phenol vapor could be recovered almost 100% simply
by re-exposing it to atmosphere for .about.60 min (or at an
elevated temperature, e.g., 60.degree. C., for only 5 min). The
recovered film demonstrated the same quenching efficiency when used
in the next cycle of the test with the phenol vapor (FIG. 10). Such
a reversible quenching can be used to distinguish phenol (if
present) from the organic amines, which otherwise exhibited almost
irreversible fluorescence quenching under the same conditions,
i.e., only .about.50% of the fluorescence could be restored even
after heating up the film overnight. The less reversibility
observed for the quenching with amines is largely due to the much
more stable chemical binding between amines and the anhydride
moiety of molecule 1. Thus, the reported fluorescence sensor system
can be typically provided as a single-use device, in the similar
manner as a pH paper or pregnancy kit, which can be used by
ordinary people without worrying about how to recover the materials
after each use.
[0080] Although the fluorescence of the nanofibers cannot be
recovered after exposed to the amines, the PTCDI materials
(molecules) can be recovered simply by redissolving the nanofibers
into chloroform, followed by appropriate purification (e.g.,
extraction with water) to remove the amines. The PTCDI molecules
thus recovered (showing again the 100% fluorescence quantum yield)
can be refabricated into the nanofibers and maintain the same
sensing efficiency for amines. To this end, the PTCDI materials are
recyclable, in contrast to the other irreversible sensor systems,
for which the sensor materials are usually unrecyclable due to
permanent chemical damage.
EXAMPLE 2
[0081] This example provides a system with increased sensitivity
for amines over Example 1 (lower detection limit). Ultrathin
nanofibers only 30-50 nm in diameter were fabricated from a
perylene based molecule,
N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide-
. The ultrathin nanofibers hereby fabricated, in comparison with
the much larger fibers of Example 1 enables enhancement in
fluorescence quenching efficiency, mainly due to the increased
surface area offered by the ultrathin nanofibers, which in turn
allows for increased vapor exposure to amines. Moreover, films
formed from thinner fibers possess increase porosity, facilitating
the expedient cross-film diffusion of gaseous species and thus
enhancing the collection and accumulation of the trace vapor
analytes, combination of which leads to unprecedented sensing
sensitivity.
[0082] The ultrathin nanofibers were prepared by a quick
crystallization process, i.e. directly injecting a good solvent
solution of the perylene monoimide into a poor solvent in a small
test tube, followed by aging. FIG. 11A shows a SEM image of the
nanofibers measured by a FEI NanoNova microscope, demonstrating
relatively uniform size and shape with diameter ranging from 30 to
50 nm. The nanofibers exhibit the same UV absorption and
fluorescence emission spectra as that of the larger fibers (350 nm
in diameter), which was fabricated through a vapor diffusion
process, i.e. about 0.2 mL CHCl.sub.3 solution of the perylene
monoimide (1.7 mM) was exposed to a methanol vapor in a closed
chamber for one day. The same spectral property (and thus
electronic structure) is indicative of the same intermolecular
organization for these two sizes of fibril structures despite of
the different fabrication methods. This simplifies the comparative
study when employing the two sizes of fibers for the vapor sensing
of amines, for which the fiber size will be the only major factor
determining the sensing sensitivity, rather than the molecular
stacking mode. The same intermolecular stacking structure of the
ultrathin nanofiber also yields the same fluorescence quantum yield
as that of the larger fibers (ca. 15%), which facilitates the
application in fluorescence sensing. FIG. 11B shows a fluorescence
microscopy image of a nanofibril film deposited on a glass
substrate, where strong red fluorescence emission of the nanofibers
can easily be visually observed.
[0083] In this example, aniline was chosen as the target vapor
analyte, mainly due to its relatively lower saturated vapor
pressure (880 ppm) compared to other organic amines, which makes it
easy to dilute the vapor down to a pressure level that matches the
detection limit for the new nanofibril sensing system as described
below. For example, 35 ppb of aniline vapor can be simply generated
by injecting 0.2 mL saturated aniline vapor into a 5 mL cuvette.
This value represents the lowest vapor pressure so far produced in
this lab, and has been used in the test of the fluorescence
quenching sensitivity of the ultrathin nanofibers. The fluorescence
quenching experiments were performed by injecting the saturated
aniline vapor into a sealed optical cell (5 mL) with the nanofibers
deposited on one inner surface. The fluorescence spectra of such a
nanofibril film (0.35 mg totally deposited) in the presence of
different pressures of aniline vapor are shown in FIG. 12A.
[0084] Dramatic fluorescence quenching (13%) was observed for the
nanofibril film after 60 s of exposure to only 35 ppb aniline
vapor. As calculated, considering both the molecular amount of the
nanofibers and aniline vapor, one aniline molecule can quench the
fluorescence emission corresponding to seven building-block
molecules within a nanofiber, i.e., the fluorescence quenching is
amplified due to the one-dimensional enhancement of exciton
diffusion along the long axis of nanofiber. Under the same
measurement condition, only ca. 4% quenching (FIG. 13A) was
observed with the larger nanofibers (350 nm in diameter), i.e. one
aniline molecule can only quench two building-block molecules
emission. The decreased quenching efficiency is likely due to the
enlarged cross-section size of the fibers, for which the exciton
diffusion is more bulk dispersed, not as confined along the long
axis as expected for the ultrathin nanofibers. One-dimensional
confined exciton diffusion is usually conducive to enhancement of
fluorescence quenching if the intermolecular energy transfer is
dominant along the long axis of nanofibers. This illustrates an
effective way to improve the quenching (sensing) efficiency simply
by decreasing the size of the nanofibers, which in turn increase
the surface area of the nanofibril film thus deposited.
[0085] It should be noted that the real sensitivity of the
nanofibril film shown in FIG. 12 should be much higher than the
measured value if taking into account the technical fact that the
small volume (0.2 .mu.L) of aniline vapor cannot be released
completely into the cuvette due to the significant absorption in
the syringe. Moreover, the smaller size of nanofibers are conducive
to enhancing the porosity of the film thus deposited, i.e.,
producing a smaller pore structure but with a more bulky inter-pore
connection. This enhanced porosity, along with the increased
surface area, not only facilitates the adsorption of amine vapor,
but also strengthen the accumulation of the amine species thus
collected from the gaseous phase.
[0086] Indeed, once the aniline molecules were adsorbed into the
nanofibril film, they usually remain condensed within the solid
phase, no release back to the gaseous atmosphere. This is
consistent with the results presented in FIG. 12B, where the
quenched fluorescence remained unchanged even 30 min after the film
was exposed to 1750 ppb of aniline vapor. In contrast, for the film
deposited from the larger fibers (diameter of 350 nm) the
fluorescence intensity tended to gradually increase after exposure
to the same vapor pressure of aniline, indicating significant
release of aniline molecules back to the gaseous phase (inset, FIG.
12B). The sustainable accumulation of gaseous analytes within the
film matrix is crucial for enabling trace vapor sensing, for which
expedient and effective collection of analyte molecules from the
atmosphere environment is often a defining factor for the sensing
system.
[0087] Technically, as small as 0.1% (or below) change in
fluorescence emission intensity can be detected by a
well-calibrated photodetector (e.g., PMT). Based on such a photon
detection threshold, one way to further improve the vapor sensing
sensitivity (or detection limit) is to increase the sigal-to-noise
ratio. Generally, the less the nanofibers are employed, the less
the quencher molecules are needed for the same percentage of
fluorescence quenching, thereby leading to enhanced sensitivity to
the trace vapor analyte. However, to maintain the sufficient
adsorption and accumulation for the trace vapor, the film deposited
from a smaller amount of fibers can maintain a sufficiently high
surface area and porosity. To this end, ultrathin nanofibers are
ideally suited for fabrication as thin films (potentially using
much less materials), while still maintain high surface area and
porosity.
[0088] FIG. 13A shows the fluorescence quenching in response to the
vapor of aniline measured for the nanofibril films deposited from
different amount of fibril materials. Under the same vapor
pressure, larger quenching percentage was observed for the film
fabricated with less amount of nanofibers. For example, under the
vapor pressure of 35 ppb, 31% of fluorescence quenching was
observed for the film deposited from 0.15 mg nanofibers, whereas
only 13% of fluorescence quenching was obtained for the film
deposited from 0.35 mg nanofibers. When decreasing the amount of
nanofibers down to 0.1 mg, the quenching efficiency was further
increased to 39% under the same condition. The increased quenching
efficiency implies direct improvement of the detection limit.
[0089] FIG. 13B shows the fluorescence quenching data fitted with
the Langmuir equation. Taking a fluorescence intensity change as
1%, the detection level for the 0.35 mg film is predicted at ca. 1
ppb, whereas for the 0.15 mg film the value could be as low as 0.1
ppb. In contrast, for the film deposited from 0.35 mg large fibers
(350 nm diameter) the detection level is up to 5 ppb. The lower
sensitivity thus observed for the large fibers is mainly due to the
intrinsic smaller surface area and lower porosity. If assuming as
small as 0.1% (or below) fluorescence quenching can be measured by
a well-calibrated photodetector (e.g., PMT), the detection limit
for the 0.15 mg film can be as low as ca. 5 ppt.
[0090] In conclusion, the fluoresence sensing sensitivity of
perylene based nanofibril films for amine vapor was largely
enhanced by decreasing the size of the nanofibers, which were
fabricated through a solution-based self-assembly processing. The
enhanced fluorescence sensing is mainly due to the increased
surface area and the enhanced exciton diffusion along the long axis
of nanofiber, along with the increased porosity intrinsic to the
film deposited from the ultrathin nanofibers. The sensing
efficiency (or detection limit) can further be enhanced by reducing
the amount of the nanofibers employed in the film.
[0091] The foregoing detailed description describes the invention
with reference to specific exemplary embodiments. However, it will
be appreciated that various modifications and changes can be made
without departing from the scope of the present invention as set
forth in the appended claims. The detailed description and
accompanying drawings are to be regarded as merely illustrative,
rather than as restrictive, and all such modifications or changes,
if any, are intended to fall within the scope of the present
invention as described and set forth herein.
* * * * *
References