Polyaniline nanoscaffolds for colorimetric sensing of biomolecules via electron transfer process

Article abstract of DOI:10.1021/la200047t

Biologically important analytes such as cysteine and vitamin-C were detected by electron transfer (ET) via naked eye colorimetric sensing using a tailor-made watersoluble vself-doped polyaniline (PSPANa) as a substrate. Monomer (N-3-sulfopropylaniline) was synthesized via ring-opening of propane sultone with excess aniline and polymerized in water using ammoniumpersulfate to obtain green water-soluble polymer. Vitamin-C (ascorbic acid) and cysteine showed unexpected sharp and instantaneous color change from blue to colorless sensing action. The stoichiometry of the analyte to polymer was determined as 3:2 and 4:1 with association (or binding) constants of K = 2.1 × 10 ³ and 1.5 × 10³ M^-1 for vitamin-C and cysteine, respectively. Efficient electron transfer from vitamin-C (also cysteine) to the quinoid unit of the polyaniline base occurred in solution; as a result, the color of the solution changed from deep blue to colorless. Cyclic voltammetry analysis of PSPANa showed the disappearance of the cathodic peak at -0.21 V upon the addition of analytes (vitamin-C and cysteine) and confirms the electron transfer from the analyte to the polymer backbone. Dynamic light scattering (DLS) and zeta potential techniques were utilized to trace the molecular interactions in the electron transfer process. DLS histograms of the polymer samples confirmed the existence of nanoaggregates of 8-10 nm in diameter. The polymers possessed typical amphiphilic structure to produce micellar aggregates which facilitate the efficient electron transfer occurred between the analyte and polyaniline backbone.

Full text of DOI:10.1021/la200047t

ARTICLE
pubs.acs.org/Langmuir
Polyaniline Nanoscaffolds for Colorimetric Sensing of Biomolecules
via Electron Transfer Process
†
,‡
M. Jinish Antony and M. Jayakannan*
†
Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology,
Thiruvananthapuram 695019, Kerala, India
‡
Department of Chemistry, Indian Institute of Science Education and Research (IISER), 900, NCL Innovation Park,
Dr. Homi Bhabha Road, Pune ꢀ 411 008, Maharashtra, India
S Supporting Information
b
ABSTRACT: Biologically important analytes such as cysteine
and vitamin-C were detected by electron transfer (ET) via
naked eye colorimetric sensing using a tailor-made water-
soluble self-doped polyaniline (PSPANa) as a substrate. Mono-
mer (N-3-sulfopropylaniline) was synthesized via ring-opening
of propane sultone with excess aniline and polymerized in
water using ammonium persulfate to obtain green water-soluble
polymer. Vitamin-C (ascorbic acid) and cysteine showed un-
expected sharp and instantaneous color change from blue to
colorless sensing action. The stoichiometry of the analyte to
polymer was determined as 3:2 and 4:1 with association (or binding) constants of K = 2.1 ꢁ 10 and 1.5 ꢁ 10 M for vitamin-C
and cysteine, respectively. Efficient electron transfer from vitamin-C (also cysteine) to the quinoid unit of the polyaniline base
occurred in solution; as a result, the color of the solution changed from deep blue to colorless. Cyclic voltammetry analysis of
PSPANa showed the disappearance of the cathodic peak at ꢀ0.21 V upon the addition of analytes (vitamin-C and cysteine) and
confirms the electron transfer from the analyte to the polymer backbone. Dynamic light scattering (DLS) and zeta potential
techniques were utilized to trace the molecular interactions in the electron transfer process. DLS histograms of the polymer samples
confirmed the existence of nanoaggregates of 8ꢀ10 nm in diameter. The polymers possessed typical amphiphilic structure to
produce micellar aggregates which facilitate the efficient electron transfer occurred between the analyte and polyaniline backbone.
3
3
ꢀ1
’
INTRODUCTION
sulfonation of emeraldine base using fuming sulfuric acid or other
2
2ꢀ27
substitution reactions.
doped polymers such as N-alkyl sulfonated monomers via
Attempts were also reported for self-
Conducting polymers are emerging as important class of
1
ꢀ4
active layers in optoelectronic devices and chemical sensors.
27
electrochemical polymerization. These approaches produced
Polyaniline, polypyrrole, polyphenylenevinylene, polythiophene,
and polyflourenes were reported for chemical sensing of bio-
molecules such as DNA, RNA, enzymes, proteins, nucleic acids,
either low molecular weight polymers or insoluble polymers
2
8,29
which hampered their complete structural characterization.
5
ꢀ8
Nanostructures of polyanilines such as nanofibers, nanospheres,
and gels were also reported to disperse the nanomaterials in
water or organic solvents so that stable films could be prepared
and so on.
Among all these conjugated polymers, polyaniline
is unique due to its good environmentalꢀchemicalꢀthermal
stability and excellent reversible non-redox acid/base doping
process. The protonation and deprotonation in the polymer
backbone is accomplished by both color change as well as solid
state conductivity. Electrochemical assays of polyaniline were
reported for detecting bioanalytes such as dopamine, glucose,
cholesterol, vitamins, and various gases such as HCl, ammonia,
1,2,30
for sensor application.
Though the nanostructures showed
improved processability, making mechanically stable polyaniline
films without losing the morphology or conductivity is still far
from the reality. Chemical or biological sensing via colorimetric
naked eye detection is an inexpensive technique and also useful
31ꢀ34
9
ꢀ18
for online monitoring in vitro and in vivo studies.
line exists in three major forms: (i) green color emerldine salt,
ii) blue color emeraldine base, and (iii) colorless leucoemerl-
Polyani-
H S, and hydrogen, and so forth.
However, the mechanism
2
of the sensing was more difficult to interpret due to the complex
surface morphology, and detailed characterization techniques
were usually required to confirm the detection pathways.
(
1
9ꢀ21
dine base form (completely reduced state with absorbance in the
Further, poor solubility of polyaniline in water or organic solvents is
always an inherent major obstacle in making the assays. To
increase the solubility of polyaniline, aryl and N-substituted
polyaniline derivatives were synthesized via postpolymerization
Received: January 5, 2011
Revised:
April 12, 2011
Published: April 25, 2011
r 2011 American Chemical Society
6268
dx.doi.org/10.1021/la200047t
|Langmuir 2011, 27, 6268–6278

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ARTICLE
3
5,36
UV-region less than 320 nm) in solution.
The change of the
electron microscopy (SEM) measurements, polymer samples were
subjected to a thin gold coating using a JEOL JFC-1200 fine coater.
The probing side was inserted into a JEOL JSM ꢀ5600 LV scanning
electron microscope for taking photographs. Transmission electron
color from blue to colorless is particularly interesting for sensing
applications, since it matches with detection capabilities of the
human eye. Color change from blue (emeraldine base) to
colorless form (leucoemeraldine base) is accompanied by both
electron as well as proton transfer. This concept is not explored
for sensing of biomolecules mainly due to two important reasons:
2
microscopy (TEM) analysis was conducted using a Tecnai 30 G S-twin
300 KV high-resolution transmission electron microscope. For TEM
measurements, a suspension of self-doped material is dispersed in
methanol and deposited on a Formvar-coated copper grid via drop by
drop addition using a glass dropper. Wide angle X-ray diffraction
(
i) lack of complete solubility of polyaniline materials in water
and (ii) mismatching of the redox potential of analytes with
polyaniline backbone. Therefore, developing new approaches for
making functionalized and water-soluble polyaniline derivatives
and exploring them for colorimetric sensing via naked eye
detection is a challenging problem to be addressed for both
fundamental understanding as well as developing new systems
for sensing of new chemical or biological analytes.
(WXRD) patterns of the finely powdered polymer samples were
recorded with a Philips analytical diffractometer using Cu KR emission.
The spectra were recorded in the range of 2θ = 0ꢀ40 and analyzed using
X'Pert software. The size determination of the polymer solution is
carried out by dynamic light scattering (DLS), using a Nano ZS-90
apparatus utilizing 633 nm red laser from Malvern instruments. UVꢀvi-
sible spectra of the polymers were recorded using an Evolution 300
UVꢀvisible instrument from Thermo Scientific Instruments. The pH
values of the samples were measured using a pH 1500 pH meter from
Eutech Instruments and calibrated using buffer solutions of pH 4, 7, and
In the present approach, a water-soluble N-substituted self-
doped polyaniline, poly-N-sulfopropane aniline (PSPA), was
synthesized via oxidative solution polymerization route. The
polymer was freely soluble in water which enabled its structural
characterization by NMR and other spectroscopic techniques.
The blue colored sodium salt of the polymer (PSPANa) was
employed as substrate for the detection of cysteine and vitamin-C.
Dynamic light scattering and zeta potential analysis were utilized
to trace the molecular interactions and polymer self-assembly in
doped and dedoped forms. It was found that the polymers exist
as nanomicellar aggregates of 8ꢀ10 nm with surface charges of
12. The thermal stability of the polymers was determined using a
PerkinElmer STA 6000 simultaneous thermal analyzer at a heating rate
of 10 °C/min in nitrogen. The instrument for thermogravimetric analysis
(TGA) was calibrated with calcium oxalate monohydrate as standard.
DSC scans of the samples were recorded with a DSC Q 20 apparatus
from TA Instruments, at a heating rate of 10 °C/min in nitrogen
atmosphere, and the instrument was calibrated using indium standard.
Cyclic voltammetry (CV) of the samples was measured by using an
Epsilon E2 cyclic voltammeter using Ag/AgCl reference electrode,
platinum counter electrode, and glassy carbon working electrode.
Synthesis of N-3-Sulfopropylaniline (SPA). Excess aniline
ꢀ
20 to ꢀ30 mV. The polymer aggregates act as nanoreactor
sites for efficient electron transfer (ET) processes. ET was
accompanied by an instantaneous sharp change from blue to
colorless for naked eye detection of biomolecules. Job’s plot was
utilized to calculate the stochiometry, and the molar ratio
method was employed to calculate association constant of
polymer þ analyte complex using the BenesiꢀHildebrand
equation. The mechanism of the electron transfer process was
further confirmed by redox potential analysis by cyclic voltam-
metry. In a nutshell, electron-transfer process was utilized in a
self-doped water-soluble polyaniline for naked eye colorimetric
sensing of biologically active molecules such as vitamin-C and
cysteine.
(5.0 mL, 0.06 mol) was taken in a 100 mL round-bottom flask, and
propane sultone (1.35 g, 0.01 mol) was added dropwise. The solution
was stirred at room temperature for 6 h. The resultant white solid was
poured into acetone, filtered, and washed with acetone until the filtrate
become colorless. The solid was dried under vacuum oven for 12 h at
50 °C. It was further purified by recrystallization from hot methanol.
1
Yield = 8.80 g (95%). H NMR (500 MHz, D
2
O) δ: 7.46 (m, 3H,
ꢀ), 2.9 (t, 2H,
CH ꢀSO H), 2.06 (m, 2H, aliphatic-H). C NMR (125 MHz, D O)
ArꢀH), 7.36 (d, 2H, ArꢀH), 3.47 (t, 2H, NHꢀCH
2
1
3
2
3
2
ꢀ
1
δ: 134.5, 130.4, 129.9, 122.3, 50.2, 47.7, 20.9. FT-IR (KBr, cm ): 1594,
1
2
5
472, 1168, 1045, 756, 695, 609. FAB-HRMS (MW = 215.06): m/z
þ
14.65 (M ). Anal. Calcd. for C H NO S: C, 50.21; H, 6.09. Found: C,
9
13
3
0.31; H, 6.01.
Synthesis of Poly-N-3-sulfopropylaniline (PSPA). N-3-Sul-
’
EXPERIMENTAL SECTION
Materials. Aniline, ammonium persulphate (APS), and propane-
fopropyl-aniline (1.00 g, 4.70 mmol) was dissolved in water (17.0 mL).
APS (1.06 g, 4.70 mmol) in water (3.0 mL) was added to monomer
solution at 30 °C, and the polymerization was allowed to proceed for 2 h
without disturbance. The green polymer solution was precipitated into
acetone, filtered, and washed with acetone until the filtrate became colorless.
sultone were purchased from Sigma Aldrich Chemicals. Vitamin-C,
sodium hydroxide, sodium carbonate, and hydrochloric acid were
purchased from Merck Chemicals (India). All the above chemicals are
used without further purification.
Measurements. NMR spectra of the compounds were recorded
The green powder was dried in a vacuum oven at 60 °C for 6 h. Yield =
1
using a 500 MHz Bruker NMR spectrophotometer in D
2
O containing
0.85 g (85%). H NMR (500 MHz, D
2
O) δ: 7.46 (m, 3H, ArꢀH), 7.37
ꢀ), 2.9 (t, 2H, CH ꢀSO H), 2
1
small amount of tetramethylsilane (TMS) as internal standard. Infrared
spectra of the polymers were recorded using a Thermo Scientific Nicolet
(d, 2H, ArꢀH), 3.48 (t, 2H, NHꢀCH
2
2
3
ꢀ
(t, 2H, aliphatic-H). FT-IR (KBr, cm ): 1579, 1493, 1401, 1163, 1045,
807, 736, 598, 522. UVꢀvisible (water, nm): 320, 415, 780, 1040.
Synthesis of Sodium Salt of Poly-N-3-sulfopropylaniline
(PSPANa). Poly-N-3-sulfopropyl-aniline (1.00 g) was dissolved in
water (1.0 mL) and treated with NaOH solution (1.0 mL, 5 M). The
solution was stirred, and the resultant blue colored solution was
precipitated from methanol. The blue powder was filtered and dried
6
4
700 Fourier transform infrared (FT-IR) spectrometer in the range of
ꢀ1
000ꢀ400 cm . The purity of the samples were determined by fast
atom bombardment high-resolution mass spectrometry (FAB-HRMS:
JEOL JSM 600). The molecular weights of the polymers were deter-
mined using an Applied Bio Instruments 4800 Plus MALDI-TOF-TOF
apparatus. For conductivity measurements, the polymer samples were
pressed into a 10 mm diameter pellet and analyzed with a four-probe
using a Keithley 6221 DC and AC current source and 2181A nanovolt-
meter. The resistivities of the sample were measured at five different
positions. Temparature dependent measurement of the polymer sam-
ples was carried out with a PID controlled heating oven. For scanning
1
in vacuum oven 60 °C for 6 h. Yield = 0.95 g (95%). H NMR (500 MHz,
D O) δ: 6.77 (t, 2H, ArꢀH), 6.4 (d, 4H, ArꢀH), 3.22 (t, 2H, aliphatic),
2
ꢀ1
2.76 (t, 2H, aliphatic), 2.3 (m, 2H, aliphatic). FT-IR (KBr, cm ): 1620,
31 04 09 ,8 6, 31 03 .6 0, 1168, 1045, 817, 736, 603, 527. UVꢀvisible (water, nm):
6
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ARTICLE
1
Figure 2. H NMR spectra of the monomer (SPA), PSPA, and PSPANa
in D O.
2
benzenoid ring aromatic protons. The three equally intense
peaks at 7.15/7.05/7.95 with 1:1:1 intensity (J = 50 Hz) were
assigned to NꢀH protons in the conducting self-doped polyani-
line. The sodium salt of the polymer (PSPANa) showed large
peak shifts compared to PSPA; the peaks corresponding to
benzonoid-aromatic peaks were shifted to the upfield region at
37ꢀ39
7
.04 and 6.65 ppm.
The broad peak that appeared at
6
.21ꢀ6.63 ppm was assigned to the aromatic protons of quinoid
rings in the dedoped form. The disappearance the NꢀH protons
(
at 7.15ꢀ7.95 ppm) and the appearance of the quionoid-ring
protons in the PSPANa directly provided evidence for the
formation of dedoped structure. The number and intensity of
protons matched with the expected structure of the compounds
13
(
see the Supporting Information for SPA). C NMR spectra of
Figure 1. Synthesis of the monomer (SPA) and self-doped polymer
PSPA.
the monomer, PSPA and PSPANa in D O showed peaks with
respect to their number of aromatic and aliphatic carbon atoms
2
(
see Supporting Information). FT-IR spectra of the monomer,
’
RESULTS AND DISCUSSION
PSPA, and PSPANa were recorded by making thin pellets with
KBr (see the Supporting Information). The two strong vibra-
tions in the monomer at 1594 and 1472 cm were assigned to
symmetric (CdC) and antisymmetric (CdC) stretching. The
peaks at 1599 and 1498 cm in the polymers were assigned to
stretching vibrations of CdC (quinoid) and CdC (benzenoid),
respectively.
The monomer (N-3-sulfopropyl aniline) was synthesized via
ꢀ
1
ring-opening of propane sultone with excess aniline and purified
by recrystallization from hot methanol (see Figure 1). The
monomer was polymerized in water using ammonium persulfate
as oxidizing agent, and the resultant polymer, poly-N-3-sulfo-
propyl-aniline (PSPA), was isolated as dark green solid. The
emeraldine salt (green solid) was found to be soluble in water
and reacted with sodium hydroxide or sodium carbonate to
produce deep blue colored polyaniline emeraldine base, poly-N-
ꢀ
1
40ꢀ42
ꢀ1
The peaks at 1350, 1250, and 1041 cm were
assigned to CꢀN, OdSdO, and CꢀH vibrations, respec-
3
0
tively. The inherent viscosity of the polymer was obtained
as 0.18 dL/g in water at 30 °C, which indicates the formation
3
0
3
-sulfopropyl-aniline sodium salt (PSPANa). Usually, polyani-
of moderate molecular weight polyanilines. To further
confirm the formation of polymers, the doped polymer was
subjected to MALDI-TOF analysis using R-cyano-4-hydroxy-
cinnamic acid as a matrix (see the Supporting Information).
The mass spectrum showed peaks at regular intervals of 213
amu corresponding to the repeating units of the polymer chain
P = (C H NO S) .
lines are not soluble in water or organic solvents which limited
30
their complete structural characterization. In the present case,
polymers in both emeraldine salt and base form have shown good
solubility in water which enabled their structural characterization
by H and C NMR. H NMR spectra of the monomer and
polymers (PSPA and PSPANa) were recorded in D O, and the
1
13
1
2
n
9
11
3
n
spectra are shown in Figure 2. The peaks at 7.46ꢀ7.36 ppm in the
monomer were assigned to aromatic protons, and their aliphatic
protons appeared at 3.47, 2.9, and 2.06 ppm. The emeraldine salt
The morphology of the PSPA was analyzed by SEM and high-
resolution TEM. The SEM images of PSPA confirmed the
existence of the flakelike morphology of the polymers in the
solid state (see Figure 3). The TEM image of the polymer also
(PSPA) showed two peaks at 7.46 and 7.37 ppm with respect to
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respectively. The higher enthalpies of the transitions for the
monomer compared to that of the polymer indicate that the
monomer is highly crystalline compared to that of the resultant
polymer structures. WXRD patterns of the polymers have
showed sharp peaks at 2θ = 4.1° (d-spacing = 21.4 Å) with
31
respect to highly ordered polymer structure. PSPA showed
sharp peaks at 2θ values from 17 to 30° with respect interchain
interaction and aromatic πꢀπ interaction. Both DSC and
WXRD analysis confirmed the existence of the highly crystalline
nature of the self-doped polyaniline; however, the crystallinity of
the monomer is much higher than that of the polymer. The
thermal stabilities of the monomer, PSPA, and PSPANa were
recorded by using a thermogravimetric analyzer under nitrogen
flow (see the Supporting Information). The thermal stability of
the materials increased in the following order: PSPA < SPA <
PSPANa. Sulfonic acids are typically hygroscopic, and their
storage and thermal stabilities are usually enhanced by convert-
ing them as sodium salts. In the present case also, as expected, the
PSPANa is more stable compared to their acids (monomer and
polymer). However, the slight increase in the thermal stability of
SPA over PSPA could be correlated to the high crystalline of the
monomer compared to the latter. IꢀV plots for the polymers
were measured using a four-probe conductivity meter, and the
polymers showed a linear increase in voltage with an increase in
current and followed typical ohmic behavior (see the Supporting
Information). Four-probe conductivities of the doped and
ꢀ
3
dedoped polymer samples were obtained as 1ꢁ 10 and 6 ꢁ
ꢀ5
1
0
S/cm, respectively, which match the values reported for
35,36
N-substituted self-doped polyaniline samples.
The polymers in both doped and dedoped forms possessed
good solubility in water, and absorption spectra were recorded
ꢀ
3
ꢀ5
for various concentrations from 10 to 10 M (see the
Supporting Information). Absorbance spectra of PSPA showed
three absorption maxima at 305, 420, and 1040 nm correspond-
ing to πꢀπ*, polaronꢀπ*, and πꢀbipolaron electronic transi-
tions, respectively. The sample PSPANa showed two peaks at
3
00 and 640 nm corresponding to πꢀπ* and exciton quinoid
2
2,40ꢀ42
transitions, respectively.
The absorption maxima of the
doped and dedoped polymer remained unchanged with the
change in concentration. This clearly supports that the electronic
structures of the self-doped and dedoped structures were stable
in water irrespective of their concentration. The polymer pos-
sessed in-built sulfonic acid groups in the chain backbone, and
therefore, the role of pH on the electronic transitions is very
important to be analyzed. For this purpose, the concentration of
ꢀ
3
PSPA was fixed as 1 ꢁ 10 M (pH = 3.2), and different
Figure 3. (a) SEM image of PSPA. (b) DSC thermograms of monomer
and PSPA. (c) WXRD of monomer and PSPA.
concentrations of base NaOH or Na CO were added to vary the
2
3
pH from 3.2 to 12 (see Figure 4). Absorbance spectra show that,
upon increasing the pH, the color of the polymer solutions
transformed from the green to deep blue. This confirmed the
change in the structure from emeraldine salt to emeraldine base
with increase in the pH. The spectra showed major changes in the
four different positions 305, 420, 640, and 1040 nm with respect
to variation in the pH. On increasing the pH from 3.2 to 12.0, the
polaron (at 420 nm) and bipolaron (1040 nm) vanish and a new
peak at 640 nm corresponding to the quinoid ring appeared. The
change in the absorbance ΔA = A ꢀ A, where A and A
confirmed the existence of the flakelike morphology (see the
Supporting Information). The tendency for the formation of
flakelike morphology in the present system is attributed to the
self-doping nature of the polymers. The presence of sulfonic acid
groups in the polymer chains induces strong interchain interac-
tions for the formation of two-dimensional (2D) flakelike growth
rather than 1D fibrous morphology. The enthalpy changes of the
polymer on heating and cooling were recorded by using a
differential scanning calorimeter (see Figure 3). The self-doped
polymer showed melting and crystallization peaks at 139 °C
0
0
correspond to the absorbance at initial and at particular pH (at
420, 640, and 1040 nm), was plotted against the pH of the
solution (showed as inset in the Figure 4). The three plots of ΔA
showed a sharp transition at pH = 8.6ꢀ9.6 with respect to their
transformation in electronic structure from emeraldine base to
(
ΔH = 1.11 J/g) and 136 °C (ΔH = 1.08 J/g), respectively. The
melting and crystallization peaks of the monomer (SPA) were
obtained at 241.2 and 166.5 °C with ΔH of 30.7 and 27.1 J/g,
6
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Figure 4. Absorption spectra of PSPA with increase in pH and photographs showing color changes with pH.
salt. The pH dependent absorbance studies revealed that the
newly synthesized water-soluble N-substituted polyaniline pro-
vides a very sharp change in the color which is very attractive for
the colorimetric sensing applications.
In order to trace the sensing ability of PSPANa, it was treated
with various biomolecules (vitamins and amino acids) and
dopants (mineral and organic acids) (see the Supporting In-
formation). As expected, dopants such as camphorsulfonic acid
high selectivity only for vitamin-C and cysteine and not for other
biomolecules.
The sensing action of vitamin-C and cysteine by PSPANa was
quantitatively detected by absorption spectra using Job's plot and
molar ratio method (see Figures 5 and 6, respectively). It is
important to mention that sharp color change was noticed for
vitamin-C in the sodium salts of polymers of NaOH and
Na CO ; however, the sensing for cysteine was found promising
2
3
(
CSA) and HCl showed color change from blue to green with
for polymer solutions obtained from Na CO compared to
2 3
respect to the transformation of emeraldine base to salt. Bio-
molecules such as folic acid (vitamin B9), glycine, and lysine did
not show any change due to weak protonation ability of their
carboxylic functional groups. Interestingly, vitamin-C (ascorbic
acid) and cysteine showed a sharp and unexpected change of
color from the blue to colorless. To further understand the sharp
color change followed by the addition of vitamin-C and cysteine
into the self-doped polymer solution, a control experiment was
carried out with unsusbtituted polyaniline emeraldine base
NaOH. Upon adding vitamin-C (also cysteine), the color change
was accompanied by disappearance of the peak at 640 nm (quinoid
ring) with an increase in the absorbance at 300ꢀ320 nm. The
spectral change at 640 nm was plotted against the mole fractions
of the constituents, and this is shown in Figure 5 as inset. The
stoichiometry of vitamin-C and cysteine with polymer was found
as 3:2 and 4:1, respectively (the reason for the difference in
stoichiometry difference is explained in the mechanism part
below). In order to determine the association constant, the
molar ratio method was executed by varying the concentration
(
PANI-EB) (see the Supporting Information). The addition of
ꢀ
4
ꢀ3
vitamin-C into PANI-EB produced a green colored solution with
respect to the formation emeraldine salt, whereas cysteine did
not show any change. It suggests that the unusual color change
from deep blue to colorless by the present polymer structure
of the analyte (from 1 ꢁ 10 to 2 ꢁ 10 M of vitamin-C and
ꢀ
4
cysteine) with fixed polymer concentration (3ꢁ 10 M) (see
Figure 6). The association constants were calculated using the
BenensiꢀHildebrand equation:
(
PSPANa) was unique to the self-doped N-alkyl polyaniline and
0
0
0
not for all emeraldine bases. Therefore, the new polymer design
provides an opportunity for sensing of biomolecules such as
vitamin-C and cysteine by simple colorimetric techniques.
Further, the selectivities of cysteine and vitamin-C compared
to other amino acids (more than 17 numbers) and other water-
soluble vitamins were also tested (see the Supporting Informa-
tion). The studies showed that the self-doped polymer possessed
1=ΔA ¼ 1=½R ꢂKΔεRS½S ꢂ þ 1=½R ꢂKΔεRS
0
0
where ΔA, [R ], [S ], and Δε are the change in absorption,
RS
total receptor concentration, total substrate concentration, and
4
3
molar absorptivities of RS (ΔεRS = ε
ε
ε ), respectively.
RSꢀ Rꢀ S
The plot of 1/ΔA versus 1/[vitamin-C] showed a straight line
plot which suggests that the polymer þ analyte followed the
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Figure 5. Change in the absorbance spectra of the polymers for the total
Figure 6. Absorbance spectra of the polymers upon addition of analytes
ꢀ3
ꢀ4
concentration of [polymer þ analyte] = 10 M in water with variation
in the chemical compositions. Insets: Job’s plots of the polymer þ
analyte complex.
into the fixed concentration of polymer, [polymer] = 3 ꢁ 10 M.
Insets: molar ratio plots (top) and BenensiꢀHildebrand plots (bottom)
of the polymer þ analyte complex.
BenensiꢀHildebrand behavior. The binding constants (K) were
were stabilized by counterion interaction of sulfonate anions
3
calculated from the intercept/slope of the plots as 2.1 ꢁ 10 and
attached in the adjacent polymer chains via electrostatic self-
3
28,41,42
1
.5 ꢁ 10 for vitamin-C and cysteine, respectively. The binding
assembly (see Figure 7).
Vitamin-C (ascorbic acid) is a
constants for vitamin-C and cysteine in the present case are
comparable with those of other sensing substrate molecules such
dibasic acid for proton transfer to bases and also good reducing
agent via transfer of electrons. In the presence of dedoped
polymer, vitamin-C undergoes redox reaction to produce dehy-
4
4
as fluorescence. To study the colorimetric sensing ability in
solid state form (for practical applications), a simple technique
was constructed for the sensing action of vitamin-C and cysteine.
A thin layer of surgical grade cotton was rolled over a plastic stick
and dipped into the PSPANa solution to make a uniform blue
coating (see Supporting Information). The blue color stick was
stable for more than 2 days without color bleaching. A half-
portion of blue stick was dipped to an analyte solution of vitamin-
C, cysteine, HCl, and CSA. The blue color immediately changed
into colorless for vitamin-C and cysteine, whereas green color
appeared for HCl and CSA solution with respect to emeraldine
salt formation. The detailed colorimetric studies in solution as
well as in the solid support (on cotton) revealed that the present
water-soluble polyaniline is unique and possessed blue to color-
less sensing ability for biomolecules such as vitamin-C and cysteine
in solution as well as in solid state.
ꢀ
þ
droascorbic acid and gives away 2e and 2H . Cysteine is
another important reducing biomolecule which undergoes self-
oxidation to produce cystine with disulfide linkage and give away
ꢀ
þ
2e and 2H . Therefore, the reaction of vitamin-C and cysteine
with N-alkyl polyaniline backbone is a typical example of the
redox process from the analyte to substrate. The addition of
vitamin-C (or cysteine) into polymer induces the following
changes: (i) protonation of sodium salts of sulfonic acids in the
alkyl side chain which further dissociates the electrostatic inter-
ꢀ
þ
action between SO3 and PhꢀN cations, (ii) transfer of the
two electrons from analyte to the dications of p-phenylenedi-
imine units in the polymer and convert the polymer chain into
completely reduced form, that is, leucoemeraldine base form. As
a net result of the proton and electron transfer from vitamin-C
(also cysteine) to PSPANa (blue color), the color of the polymer
solution changes from blue to colorless. Since the kinetics is very
The mechanism for the sensing of vitamin-C and cysteine is
given in Figure 7. Chemical structures of self-doped polyaniline
N-alkylated polyanilines are different from those of normal
polyanilines, and they exist as the emeraldine salt (I) consisting
6
ꢀ1 48
fast (observed in the range of kET = 4 ꢁ 10 s ), it
accompanies the color change instantaneously for colorimetric
detection by naked eye. The higher stoichiometry (4:1, in moles)
of cysteine could be explained from the mode of chemical reaction it
undergoes during the sensing process (see Figure 7). Two cysteine
45ꢀ47
of cation radicals of p-phenylenediamine.
Addition of base
results in the formation of dedoped form of emeraldine base (II)
consisting of dications of p-phenylenediimine. The dications
þ
molecules are oxidized to form one cystine molecule to give 2H
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Figure 7. Mechanism of colorimetric sensing in self-doped N-substituted polyaniline with analytes.
ꢀ
and 2e . On the other hand, in the case of vitamin-C, each
molecule undergoes oxidation to dehydroascorbic acid (DHAA)
an anodic oxidation peak at 0.46 mV in the anodic sweep and a
reversible reduction peak at ꢀ0.21 V in the negative cathodic
þ
ꢀ
49,50
and 2H and 2e . This further reflects the high detecting ability
of vitamin-C and higher association constant compared to that of
cysteine (see insets in Figure 5). To understand the ET process,
the oxidation and reduction potential of PSPANa and its com-
plexes with vitamin-C and cysteine were determined by cyclic
voltammetry in water using glassy carbon working electrode (see
Figure 8). All samples are recorded in the range of þ1.2 to ꢀ1.2 V
at a scan rate of 50 mV/s. Bases (NaOH and Na CO in water)
region.
Under the same sweeping conditions, the oxidation
potential of vitamin-C and cysteine were found as 0.11 and 0.54
V, respectively (see the Supporting Information). Cyclic voltam-
mograms of vitamin-C þ PSPANa and cysteine þ PSPANa
complexes showed complete disappearance of the cathodic peak
at ꢀ0.21 V. The disappearance of the cathodic peak at ꢀ0.21 V
upon the addition of analytes (vitamin-C and cysteine) revealed
the electron transfer from the analyte to the polymer backbone
and supports the mechanism proposed in Figure 7. To further
investigate the electron transfer process on the conductivity of
the samples in the solid state, a 10 mm diameter pellet of the
2
3
act as electrolytes to retain the same experimental procedure
used for sensing (little excess of the analyte is used to stabilize the
signal in the CV). The cyclic voltammogram of PSPANa showed
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ARTICLE
Figure 8. Cyclic voltammograms of PSPANa, PSPANa þ vitamin-C,
and PSPA þ cysteine.
PSPANa was made and subjected to sensing studies. The blue
color pellet was dipped into vitamin-C (or cysteine) solution, and
the resultant pellet was subjected to four-probe conductivity
measurements. The conductivity of the PSPANa pellet changed
from 6 ꢁ 10 to 2.5 ꢁ 10 and 1.5 ꢁ 10 S/cm followed by
dipping in vitamin-C and cysteine, respectively. The decrease in
the conductivity values confirmed the formation of reduced
leucoemeraldine base form through electron transfer process as
shown in Figure 7. It confirmed that the water-soluble self-doped
polyaniline is a very unique and efficient sensor material for
detecting biomolecules such as vitamin-C and cysteine via electron
transfer process.
Figure 9. DLS histograms of the PSPA, PSPANa, PSPA þ vitamin-C,
ꢀ4
and PSPA þ cysteine in water at 25 °C. [Polymer] = 4 ꢁ 10 M and
ꢀ
5
ꢀ6
ꢀ6
ꢀ3
[
analyte] = 1 ꢁ 10 M.
interactions, the polymer solutions in doped (PSPA) and de-
doped (PSPANa) forms were subjected to dynamic light scatter-
ing (DLS) coupled with zeta potential measurements. DLS
histograms of PSPA, PSPANa, PSPANa þ vitamin-C, and
PSPANa þ cysteine are shown in Figure 9. Polymer samples
in doped and dedoped forms showed uniform distribution of
aggregates with average sizes of 8 and 10 nm, respectively. DLS
analysis was also done for a stored polymer solution over a period
of 1 week; the polymer did not phase separate, and the results
were almost reproducible. The polymers possessed typical
amphiphilic structure in which the aromatic backbone and alkyl
sulfonic acid groups act as hydrophobic and hydrophilic parts.
Nanomicellar aggregates form a coreꢀshell architecture in which
the hydrophilic sulfonic acid part is kept outside facing the
aqueous environment and the polyaniline backbone provides
the hydrophobic core. Upon dedpoing the PSPA with sodium
hydroxide, the outer sulfonic acid groups converted into sodium
sulfonate and the polyaniline backbone became quinonid; how-
ever, the hydrophilicity of the core and outer shell was retained.
Therefore, during chemical doping/dedoping processes, the
Electron transfer is a short-range order process typically
occurred in a closed vicinity of the electron donor and acceptor
species. In biological systems, such as flavoenzymes, the electron
transfer is efficient because the protein folding creates close
packing of the reaction site for electron transfer between donors
5
1ꢀ55
and acceptors.
In synthetic molecules, close contact be-
tween donor and acceptor units is achieved either via covalent
linkages or in the coordinated crystal structures. It is very
surprising to notice that, in the present case, the ET process
was efficient between the conducting polymers with analytes
(
vitamin-c or cysteine) which are not chemically interconnected
together. Therefore, it is very important to trace the molecular
interactions between vitamin-C (and cysteine) with PSPANa to
understand the ET process. In order to trace the molecular
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Figure 10. Schematic model for the colorimetric sensing via ET process in the polymer nanoaggregates.
like the doped and dedoped polymers in the presence case.
Electrically charged species tend to move under the influence of
an electric field, and the electrical potential at the surface of the
sphere of radius a and carrying charge q is ζ = q/4πεa, where ζ
and ε are zeta potential and permittivity of the medium in which
56
they are immersed, respectively. The effect of concentration on
the size of the micellar aggregates and their zeta potential are
shown in Figure 11 (for DLS histograms, see the Supporting
Information). The size of the nanoaggregates in PSPANa was less
influenced by concentration variation, and the values were found
in the range of 10 ( 2 and 8.0 ( 2 nm for both doped and
dedoped structures, respectively. The aggregate sizes of the
PSPANa þ vitamin-C and PSPANa þ cysteine complexes were
found to be much higher than that of their homopolymer
aggregates. The addition of analytes into the polymer solution
increases the sizes of aggregates from 10 to 20 and 30 nm for
vitamin-C and cysteine, respectively. However, we need more
experimental evidence to confirm the occupancy of analyte in the
hydrophobic cavity of the nanoaggregates. The analytes are
typical organic molecules, and they can preferentially occupy
the hydrophobic cavity provided in the nanoaggregates, and
these phenomena attributed to the increase in the size of the
aggregates. The zeta potential of the polymer nanoaggregates
possessed negative charges; however, their magnitude varies with
the type of structure. All three plots of zeta potential versus
concentration showed a nonlinear trend with a break point at
ꢀ
4
4
.0 ꢁ 10 M, which is very close to the critical micellar concen-
tration (CMC) of these nanoaggregates. Since polymer chains
possessed long-range chain interactions, unlike in the case of
small molecule surfactants, it may contain critical association well
below the CMC. Nevertheless, the detailed DLS and zeta
potential studies confirmed that the addition of vitamin-C or
cysteine did not destabilize the polymer nanoaggregate and the
aggregates are stable for a wider concentration range for the
detection process. Thus, the newly synthesized self-organized
nanoaggregates provide reaction sites in which electron transfer
occurred between the analyte and polyaniline backbone for
colorimetric sensing of vitamin-C and cysteine.
Figure 11. Size and zeta potentials of PSPANa, PSPANa þ vitamin-C,
and PSPANa þ cysteine at various polymer concentrations for fixed
ꢀ3
[
analyte] = 1 ꢁ 10 M.
polymer chains were retained in the nanoaggregate form without
affecting the hydrophilic/hydrophobic nature of the aggregates.
A schematic model for the polymer nanoaggregates is shown in
Figure 10.
Zeta potential measurement is very important tool for under-
standing the solution dynamic of charged particles or aggregates,
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ARTICLE
’
CONCLUSION
In conclusion, a water-soluble self-doped polyaniline was
photographs of the vials showing the selectivity of L-cysteine over
other L-amino acids; photographs showing selectivity of vitamin-
C by PSPANa and absorption spectra with addition of different
water-soluble vitamins; cyclic voltammetry of vitamin-C and
cyteine. This material is available free of charge via the Internet
at http://pubs.acs.org.
synthesized and successfully utilized as substrate for detection
of biomolecules such as vitamin-C and cysteine via electron
transfer in nanoaggregates. The main outcomes of the present
investigation are as follows: (i) N-propylsulfonic acid aniline
monomer was synthesized via ring-opening of sultone with
aniline, which was further polymerized via chemical oxidative
route to produce completely water-soluble polyaniline; (ii) the
structures of the polymers in doped and dedoped forms were
’
AUTHOR INFORMATION
Corresponding Author
*
E-mail: jayakannan@iiserpune.ac.in. Fax: 0091-20-25898022.
1
13
characterized by both H and C NMR, and the molecular
weights were determined by viscosity and MALDI-TOF techni-
ques; (iii) pH dependent absorbance studies revealed that the
newly synthesized water-soluble N-substituted polyaniline pro-
vides very sharp color change which is very attractive for the
colorimetric sensing applications; (iv) various dopants such as
camphorsulfonic acid (also HCl) showed expected color change
from blue to green with respect to the transformation of
emeraldine base to its salt; (v) the unusual color change from
deep blue to colorless by PSPANa was unique to the self-doped
N-alkyl polyaniline structure and provides new opportunity for
sensing of biomolecules such as vitamin-C and cysteine by simple
colorimetric techniques; (vi) Job’s plots for vitamin-C and
cysteine established the stoichiometric composition of vitamin-
C and cysteine with polymer as 3:2 and 4:1, respectively; (vii) the
binding constants were determined by molar ratio method using
’
ACKNOWLEDGMENT
We thank the Department of Science and Technology,
New Delhi, India under the NSTI Programme-SR/S5/NM-06/
007 and SR-NM/NS-42/2009 for financial support. M.J.A. thanks
UGC-New Delhi, India for Senior Research Fellowship (SRF).
2
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