Article pubs.acs.org/JPCC
Catalytic Reactions on the Surface of Ag Nanoparticles: A Photochemical Effect and/or Molecule Property? Ridhima Chadha, Nandita Maiti,* and Sudhir Kapoor* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *
ABSTRACT: Using time-dependent surface-enhanced Raman scattering (TDSERS), we demonstrate the surface-catalyzed oxidation of 3-hydroxy anthranilic acid (HAA) to its azo derivatives. No external source either in the form of laser excitation or heat was required for the surface-catalyzed reaction, which clearly established that thermal energy alone was sufficient. But the possibility of enhanced reaction rate due to the surface plasmon resonance could not be ignored. To mimic in vivo conditions that prevail in healthy and unhealthy cells, SERS measurements were recorded under different environmental conditions. It is shown that the surface-catalyzed azo formation and its isomerization were strongly dependent on the external conditions, namely, temperature, pH, and environment. A plausible mechanism based on electron transfer to the adsorbed O2 is proposed for the trans−cis isomerization. This study can lead toward a new strategy of surface-catalyzed reactions not only for the synthesis of azo dyes but also to distinguish between the normal and abnormal cells.
1. INTRODUCTION The control of chemical reactions on nanoparticle surfaces plays an important role in the photochemistry of surfaces and in building molecular devices. These surface photochemical reactions have been exploited recently using various techniques, viz., scanning tunneling microscopy (STM), transmission electron microscopy (TEM), near field scanning optical microscope (NSOM), surface-enhanced Raman scattering (SERS), etc. Nanoscale control of reversible chemical reactions, viz., the polymerization and depolymerization between C60 molecules, was studied experimentally by application of negative and positive bias voltage.1−4 The quantitative measures of the nanoscale wear of silicon, a material relevant to small-scale devices, was studied using in situ TEM.5 Surface photochemical reactions on nonconductive surfaces have been studied using NSOM.6 Over the years, SERS has proven to be one of the most sensitive spectroscopic techniques that provides greatly enhanced Raman signal7,8 from Raman-active analytes that have been adsorbed onto certain specially prepared nanometer-sized metal nanostructures. The enhanced Raman signals in SERS are generally explained on the basis of two separate mechanisms, namely, the electromagnetic (EM)9 effect and the chemical (CM)10−12 or charge-transfer effect. Both effects (EM and CM) occur simultaneously and contribute toward enhancement of the Raman signals, and it is usually difficult to separate out these two effects in the SERS spectra. Nevertheless, it is accepted that the contribution of CM effect in the overall SERS spectra is much less as compared to the EM effect. Recently, it has been shown that by using a spacer or increasing the chemical bonding between the adsorbed molecule and metal nanoparticles (NPs) surface it is possible to separate out the chemically enhanced modes from © XXXX American Chemical Society
the electromagnetically enhanced modes in the SERS spectrum.13 The SERS technique has been widely exploited in identifying biomolecules, drugs, and proteins as it provides information about the interaction of the analytes with the metal surface, i.e., the binding sites and the orientation over the metal surfaces.14−25 SERS is also being exploited recently for studying surface-catalyzed reactions. The interfacial surface-catalyzed reactions on metal nanostructures could be explained in terms of new vibrational features that are not due to the chromophores present in the original molecule. Extensive experimental studies have been carried out in recent times on SERS of 4-aminothiophenol (PATP) adsorbed on different nanosubstrates. The main interest in this compound was due to the enhancement of three Raman peaks at 1140, 1391, and 1440 cm−1 which was interpreted initially to arise due to Herzberg−Teller contribution to the chemical mechanism.26 However, ambiguity in the interpretation of the abovementioned SERS peaks resulted in many researchers focusing their attention toward explaining the origin of these SERS peaks in PATP.27−38 It is worth mentioning here that the antioxidant/biological properties of molecules having aniline, phenols, and thiols are due to the presence of labile hydrogen atoms. Biological molecules, having groups with labile atoms, can undergo catalytic/photolytic reactions on the surface of coinage metal NPs. For this reason, we have selected a compound, 3-hydroxy anthranilic acid (HAA), which is an important biological molecule having various metal binding groups. Due to the Received: September 11, 2014 Revised: October 21, 2014
A
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presence of −NH2, −OH, and −COOH groups at ortho positions, it can easily undergo intramolecular hydrogen bonding. HAA, an aminophenol, is a tryptophan metabolite that plays anti-inflammatory and neuroprotective roles during inflammation.39 It may also have potent actions on cell function in the nervous and immune systems.40 Thus, with this important biological molecule, HAA, we have tried to address the following questions: (i) whether surface-catalyzed reactions are molecule specific and (ii) whether these reactions depend on the experimental conditions such as thermal energy, photon energy, pH, and environment (air, N2 and O2). In this paper, we have addressed the above-mentioned points by demonstrating in situ surface-catalyzed oxidation of HAA to form the trans isomer of 2,2′-dihydroxy-6,6′-dicarboxylic-azobenzene (DHDCAB) on different kinds of Ag NPs and its subsequent isomerization to the cis form using TDSERS and TDUV−vis absorption spectroscopy.
The pKa of HAA is known to be 5.20 and 10.09. Hence, it exists mainly as anthranilate (carboxylate ion) in aqueous solution at pH 7.8. The vibrational features observed in Raman spectra were interpreted using the theoretically calculated Raman spectrum of HAA using the density functional theory (DFT) with B3LYP functional44 and aug-cc-pvdz basis set for HAA and the carboxylate ion of HAA (HAA-CO2−) using Gaussian 03 program.45 The 6-31G and 6-31G* basis sets were used for optimization and frequency calculation of the trans and cis isomers of DHDCAB.
3. RESULTS AND DISCUSSION The Raman spectrum of HAA in the neat solid is shown in Figure 1a. The assignments of the Raman bands to the
2. EXPERIMENTAL SECTION 3-Hydroxy anthranilic acid (HAA), AgNO3, Ag2O, tetrachloroaurate, sodium borohydride, and sodium citrate used in this study were purchased from Aldrich and used without further purification. Silver colloid was prepared by the chemical reduction of AgNO3 with NaBH4 using the method of Creighton et al.41 The silver colloid prepared using NaBH4 reduction method was yellow in color and was stable at room temperature for several weeks. Colloidal silver was also prepared by the reduction of silver nitrate with sodium citrate using the method of Lee and Meisel.42 HAA was added to the colloidal silver solutions prepared using NaBH4 or sodium citrate reduction and the resultant solutions with pH of ∼7.8 and 5.7 were characterized using UV−vis absorption and Raman spectroscopic techniques. The pH of the solution was varied by adding NaOH solution or dilute HCl. Colloidal gold nanoparticles were prepared by the reduction of tetrachloroaurate with sodium citrate using the Turkevich method.43 UV−vis absorption spectra were recorded using a Jasco V-650 spectrophotometer. Raman and SERS spectra of HAA were recorded at room temperature using the 514 nm line, from an Ar+ ion laser (35-LAP-431-230, Melles Griot). In order to record the excitation wavelength-dependent SERS spectra, 488 nm (Ar+ ion), 532 nm (diode-pumped solid state Nd3+:YAG), and 633 nm (HeNe) laser lines were used. The sample solutions were taken in a standard 1 × 1 cm2 cuvette or on glass slides, and the Raman scattered light was collected at 180° scattering geometry or with a 50X LWD (long working distance) objective and detected using a CCD (Synapse, Horiba Jobin Yvon) based monochromator (LabRAM HR800, Horiba Jobin Yvon, France) together with an edge filter, covering a spectral range 200−1800 cm−1. The spot size on the sample was ∼0.5 mm in diameter, and the laser power at the sampling position was 8, 10.2, 50, and 12.7 mW for the excitation wavelengths of 488, 514, 532, and 633 nm, respectively. The power density on the sample was 8.16, 10.40, 51.0, and 12.95 W/cm2 for 488, 514, 532, and 633 nm, respectively. The Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm−1. The electrospray ionization mass spectrometry (ESI-MS) (MicroTOF Q 11, Bruker Daltonics, Germany) was used to acquire the mass spectrum of the silver colloid with added HAA in negative ion mode. The details of the recording conditions are mentioned in the Supporting Information.
Figure 1. (a) TDSERS spectra of HAA (10−4 M) in the range 400− 1800 cm−1 at pH 7.8 and (b) formation of the trans NN stretching (str) band at 1444 cm−1 as a function of time and relative decrease in intensity of the trans NN str (1444 cm−1) with respect to the cis NN str (1515 cm−1) with increasing time (10% error bars have been included).
stretching and bending vibrations of the phenyl (ph) ring, carboxylate (CO2−), and the amino (NH2) groups are based on comparison of the Raman spectrum of the solid with the B3LYP/aug-cc-pvdz calculated vibrations of HAA and are tabulated in Table 1. The optimized structure of HAA along with atom numbering is shown in Figure 2. The B3LYP/augcc-pVDZ calculated Raman spectrum of HAA along with the Raman spectrum of solid HAA is also shown in Figure 3. The Raman spectrum of solid HAA exhibits four strong bands in the frequency region 400−1800 cm−1 (Figure 1a). These Raman bands are observed at 1346, 1299, 783, and 627 cm−1. The 1346 and 1299 cm−1 bands are attributed to CCO2 stretch (str) and CNH2 coupled to ph CH bend, respectively, and the vibrations at 783 and 627 cm−1 are assigned to ring breathing and ring bending. Medium intensity bands are observed at 1551, 1414, 1176, 1073, 505, and 457 cm−1. The 1551 and 1414 cm−1 bands are assigned to ph CC str and are coupled to NH2 scissoring (sci) and ph CH bend, respectively. The 1176, 1073, 505, and 457 cm−1 Raman bands are assigned to ph OH bend, ph CH bend, ph ring bend, and NH2 rock. Few weak bands are observed at 1652, 1619, 1222, 1098, 1010, 908, 821, and 588 cm−1. The weak bands at 1652 and 1619 cm−1 are assigned to CO str and NH2 sci, respectively. The Raman bands at 1222, 1098, 1010, 908, 821, and 588 cm−1 are attributed to COH str, ph CH bend, ph CH twisting, ph ring bend, ph ring distortion, and ph OH bend, respectively. The Raman spectrum of HAA in aqueous solution could not be recorded due to its low solubility. In fact, HAA even at 10−3 M concentration is sparingly soluble in water, and hence the B
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Table 1. Assignment of Raman (Solid), SERS, and B3LYP/ aug-cc-pvdz Calculated Vibrations (in cm−1) for 3-Hydroxy Anthranilic Acid (HAA) and Its Carboxylate Ion (HAACO2−)a Raman solid
calcd HAA
calcd (HAACO2−)
1652w
1663
1619w
1619
1661 1635
1551m
1586
1533
1414m
1421
1346s
1326
1387 1351 1298
1299s
1291
1270
1222w 1176m
1262 1197
1229 1168
1276m 1249w 1162w
1135
1124w
SERS 1644w 1618m 1594w 1560w 1515s 1487w 1444s 1392s 1330w
1098w 1073m 1010w 908w 821w
1095 1095 971 901 849
1074 965 906 856
1078w 977w
783s
706
766
785m
627s
624
634
634m 592m
588w
574
578
505m
499
492
457m
417
425
834w
approx assignments CO str ph CC str, ph OH bend NH2 sci, ph CC str, CO str, ph CC str NH2 sci, ph CC str, NN str (cis) ph CC str, NN str (trans) NN str (trans) ph CC str, ph CH bend sym CO2− str ph CH bend, CCO2 str ph CH bend, CNN bend CNH2 str, ph CH bend CNN bend COH str, ph CH bend ph OH bend, ph CH bend, (CO11H bend) CCO2 str, NH2 rock, ph CH bend ph CH bend ph CH bend ph CH twisting ph ring bend, NH2 rock ph ring distortion NH2 wagging ring breathing, CO2 bend, CO11H bend ring bending, CO2 bend CNNC torsion (cis) ph OH bend, NH2 rock, CO11H bend ph ring bend, ph CH bend, ph OH bend NH2 rock, ph OH bend
Figure 3. Raman and SERS spectra (pH 7.8) of HAA compared with the computed Raman spectra of HAA, HAA-CO2−, trans-DHDCAB, and cis-DHDCAB.
The SERS spectrum recorded immediately upon addition of HAA to silver colloid with pH ∼ 7.8 is shown in Figure 1a, and the SERS frequencies are displayed in Table 1. For a comparison of the SERS vibrations, the calculated Raman frequencies of the carboxylate ion of HAA (HAA-CO2−) are also included in Table 1 along with the calculated Raman frequencies of HAA. The computed Raman spectrum of HAACO2− with B3LYP/aug-cc-pVDZ is also included in Figure 3. It was observed from Figure 1a that, immediately upon adsorption, HAA shows some features similar to its Raman spectrum in neat solid. Additional peaks are observed at 1392, 1444, 1487, and 1594 cm−1. The strong SERS band at 1392 cm−1 can be assigned to the CO2− symmetric (sym) str. This strong Raman band is usually observed in the SERS spectra where the probe molecule binds to the metal NPs surface through its carboxylate group.21,22 The TDSERS spectrum of HAA (10−4 M) in silver colloid with the resultant pH of 7.8 in air for 400−1800 cm−1 region, recorded at 514 nm excitation wavelength, is presented in Figure 1a. The TDSERS spectra show the steady growth of the Raman bands at 1444 and 1487 cm−1. The initial growth of the intense 1444 and 1487 cm−1 Raman bands is followed by the disappearance of the band at 1618 cm−1 that is assigned to the NH2 sci mode of HAA (Table 1). The disappearance of NH2 sci mode shows that HAA has undergone some structural transformation. Similar to SERS studies of PATP and its azo derivative,27−31 the appearance of the two new peaks at 1444 and 1487 cm−1 are attributed to NN str (trans) and ph C C str coupled to NN str (trans), respectively, of the azo derivative of HAA, i.e., DHDCAB. The TDSERS spectra also show the growth of 1276, 1594, and 1644 cm−1 bands that are assigned to the CNN bend, CO str, and ph CC str coupled to ph OH bend, respectively. Within 15 min of the initial surface-catalyzed reaction in air, gradual increase in the Raman bands at 1515 and 592 cm−1 is observed. As observed in the case of surface-catalyzed isomerization of azobenzene,46−48 these Raman bands at 1515 and 592 cm−1 can be attributed to the NN str and CNNC torsion of the cis isomer of DHDCAB. The formation of the trans isomer of DHDCAB and its decay to form the cis isomer as a function of time is shown in Figure 1b. DFT (B3LYP with 6-31G and 6-31G* basis sets) calculations were carried out for the trans and cis isomers of
a
Abbreviations used: s, strong; m, medium; w, weak; sci, scissoring; str, stretch; ph, phenyl; sym, symmetric.
Figure 2. Optimized structure of HAA along with atom numbering.
normal Raman spectrum shows poor signal-to-noise ratio. Due to this reason, it was not possible to study the effect of pH variation on the normal Raman spectral pattern of HAA. C
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DHDCAB. The relative energies for the trans and cis isomers of DHDCAB at B3LYP/6-31G and B3LYP/6-31G* basis sets are ∼0.3 eV with the trans isomer being more stable than the cis form. The Raman vibrations were computed at the optimized geometry for both the isomers with 6-31G and 6-31G* basis sets. The B3LYP/6-31G computed Raman spectrum for transDHDCAB and cis-DHDCAB was found to be in good agreement with the TDSERS spectrum obtained after 2 and 24 h of the initial surface-catalyzed oxidation of HAA and is shown in Figure 3. In the TDSERS spectra of HAA, the weak peak observed at 1249 cm−1 (assigned to the COH str) appears initially, but it slowly disappears with time. The strong Raman band at 1392 cm−1 (CO2− sym str) slowly goes down in intensity with time (4 h) and completely disappears after 24 h of the initial surface-catalyzed reaction. This is due to the fact that initially the trans isomer of DHDCAB remains bound to the surface via its carboxylate group, but upon isomerization to the cis form, the binding to the metal surface is through the NN (azo) group and not through the carboxylate group, leading to intense Raman signature at 1515 cm−1 and complete disappearance of the 1392 cm−1 peak. Thus, the dynamic changes observed in the TDSERS spectra (Figure 1a) clearly demonstrate the surface-catalyzed oxidation of HAA to form the trans isomer of DHDCAB that subsequently isomerizes to the cis form. The surface-catalyzed oxidation of HAA in air is almost complete within 24 h and reaches equilibrium between the trans and cis isomers of DHDCAB. It is worth mentioning that although we expected to see some changes in the NH str region (3300−3500 cm−1) due to azo formation, but at HAA concentration of 10−4 M, this Raman band was completely masked by the huge OH str band of water. The probable orientation of the molecule on the Ag NP surface can be known from the SERS intensities. According to the surface selection rules, normal modes of adsorbed molecules involving changes in molecular polarizability with a component perpendicular to the surface are subject to the greatest enhancement. The enhanced intensities of the 1619 and 1392 cm−1 vibrations are assigned to the NH2 sci and CO2− sym str. This enhancement in intensities indicates that, immediately upon addition of HAA to the Ag NP surface, it gets adsorbed through the CO2− and NH2 groups leading to a titled orientation that results in the oxidation of HAA. The azo derivative once formed (enhancement in the 1444 and 1515 cm−1 bands) probably orients parallel to the Ag NP surface and remains flat over the surface. The TDSERS spectra of HAA adsorbed on Ag NPs stabilized by sodium citrate with the resultant pH of 5.7, recorded at excitation wavelength 514 nm, is shown in Figure 4. The figure clearly shows that, at pH 5.7, at zero time, relatively more of the cis form of DHDCAB (1510 cm−1) is observed as compared to the trans isomer (1442 cm−1). At this pH, the surface-catalyzed azo formation and isomerization proceed much faster than at pH 7.8, and the reaction is almost complete and reaches equilibrium between cis and trans isomers within 1 h. As shown in the figure, the vibrational features in the TDSERS spectra remain almost similar with an overall increase in intensity as a function of time. The effect of temperature on the thermally equilibrated trans and cis isomers of DHDCAB was investigated on the Ag NPs stabilized by sodium citrate at pH 5.7 (Figure 5a). The SERS spectra recorded at 514 nm with increasing temperature showed a relative increase in the intensities of the 1442 cm−1 [NN str (trans)] Raman band as compared to the 1510 cm−1
Figure 4. TDSERS spectra of HAA (10−4 M) at pH 5.7 in the range 1200−1700 cm−1.
[NN str (cis)] peak indicating an increase in population of the trans isomer of DHDCAB with respect to the cis isomer. The relative population of the trans isomer relative to its cis conformer increases with increasing temperature with the equilibrium shifting toward the thermally more stable trans azo derivative of HAA. The 1330 cm−1 band (ph CH bend coupled to CNN bend) related to the trans isomer of DHDCAB also gains intensity while the 1276 cm−1 band (CNN bend) corresponding to the cis form becomes less intense with increasing temperature. In order to study the effect of pH on the azo formation and its isomerization, SERS experiments at 514 nm excitation were carried out at different pH on the Ag NPs stabilized by sodium citrate (Figure 5b). In silver colloid, immediately upon addition of HAA at pH 5.7, strong peaks are observed at 1510 cm−1 [NN str (cis)] and 1276 cm−1 (CNN bend) with weak bands appearing at 1442 cm−1 [NN str (trans)] and 1330 cm−1 (ph CH bend coupled to CNN bend). In the acidic pH range varying from ∼3 to 6 (Figure 5b), the relative ratios of the 1510 and 1442 cm−1 Raman bands show that the population of the cis isomer of DHDCAB is much higher than that of the trans isomer. It is worth mentioning here that trans−cis isomerization showed pH dependence, and it was reversible within the pH range ∼3−10. It can be visualized that protonation of NH2 would inhibit azo formation. Indeed, similar observation was made when SERS spectrum was recorded at pH 2.4 as shown in Figure 5b. In this study, the observation of more cis isomer of DHDCAB over Ag NPs at acidic pH 3−6 could be due to faster isomerization49 because of lower potential energy barrier between the two isomers. The surface-catalyzed reaction of HAA on Ag2O was also investigated in order to know the exact mechanism, and the TDSERS spectra recorded at 514 nm are shown in Figure 5c. The figure shows that surface-catalyzed azo formation and isomerization reaction start almost immediately when HAA is added to Ag2O surface (powder). With increasing time from 5 to 30 min, the cis form gets converted to the trans DHDCAB isomer due to local heating. Recently, the role of O2 in the surface-assisted catalytic reaction was shown by Wu and co-workers.29 They suggested that surface plasmon assisted electron transfer from metal to 3 O2 leads to the creation of 2O2− (anion of oxygen) that gets adsorbed over the surface of metal clusters, thus forming Ag2O and Au2O. The activation of 3O2 by surface plasmon thus plays D
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Figure 5. SERS spectra of HAA with (a) increasing temperature, (b) change in pH, and (c) TDSERS spectra of HAA on Ag2O surface.
Figure 6. TDSERS spectra of HAA (10−4 M) at pH 5.7: (a) N2-bubbled and (b) O2-saturated.
Figure 7. TDSERS spectra of HAA (10−4 M) at pH 7.8: (a) N2-bubbled and (b) O2-saturated.
a crucial role in the oxidation of PATP to p-p′-dimercaptoazobenzene (DMAB). In this study, in order to check the effect of O2 on the surface-catalyzed oxidation of HAA and its isomerization, the TDSERS was monitored under different environmental (N2, O2, and N2O) conditions. The silver colloid containing HAA was saturated with either N2, O2, or N2O and the sample holder with the lid sealed tightly with parafilm. The TDSERS spectra recorded at 514 nm under N2-bubbled and
O2-saturated conditions at pH 5.7 and 7.8 are shown in Figures 6 and 7, respectively. The TDSERS spectra recorded at 514 nm under N2O saturated condition at pH 5.7 and 7.8 are shown in the Supporting Information, Figure S1. On comparing these results with the TDSERS spectra recorded in air, it is observed that, in an aerated solution, at acidic pH contrary to pH ≥ 7.8, intensity of cis isomer is much more as compared to that of the trans isomer. Also, at pH ≥ 7.8, the formation of cis isomer is E
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water.50 It is noteworthy to mention here that when the asprepared Ag NPs were used as substrate for HAA, the observed TDSERS spectrum (Supporting Information, Figure S2) was almost similar to the results obtained in air (Figure 1a). This experiment provides further evidence that the support plays a key role in surface-catalyzed oxidation of HAA and its isomerization. In order to ensure whether the surface-catalyzed oxidation of HAA is due to its molecule property, i.e., if the functional groups of the adsorbed molecule have any role to play, the TDSERS spectra were also recorded for anthranilic acid. It is noteworthy that no new spectral feature due to the formation of azo derivative was observed in the case of anthranilic acid. In HAA, the presence of −OH and NH2 groups in ortho position results in intramolecular hydrogen bonding that leads to the weakening of the N−H bond and thus facilitates the formation of the azo derivative of HAA. Earlier reports on TDSERS measurements suggested that surface-catalyzed reaction depends on incident excitation wavelength,36,37 thus providing evidence for the role of surface plasmons in such reactions. However, in the present study it was observed that, under ambient conditions, with the increase in reaction time, the Raman peaks at 1444 cm−1 (trans azo) and 1515 cm−1 (cis azo) show relative changes in the intensity. This study infers that the surface-catalyzed oxidation of HAA occurs even without laser excitation showing that the potential energy barrier for the reaction is relatively low. Nevertheless, to confirm this, the SERS spectrum of HAA at pH 7.8 was monitored at different excitation wavelengths and is shown in Figure 8a. The excitation wavelengths used to record the SERS spectrum are also marked in the TDUV−vis absorption spectrum (Figure 9). The SERS spectrum recorded with excitation wavelengths, 488, 514, 532, and 633 nm in the region 400−1800 cm−1, falls in the wavelength ranges 497.7−534.9, 524.8−566.4, 543.6−588.3, and 649.4−714.4 nm, respectively. These SERS spectra show almost similar features, but with reduced intensity at 633 nm. The reason for reduced SERS intensity at 633 nm excitation is the fact that the spectrum (400−1800 cm−1) falls in the wavelength range 649.4−714.4 nm which lies along the falling absorption edge. Figure 9 clearly shows that the SERS spectra recorded at shorter excitation wavelengths of 488, 514, and 532 nm are in resonance with the absorption band around 500−650 nm. The differences observed in the relative intensity of peaks corresponding to the trans and cis isomers in the excitation wavelengthdependent SERS spectra indeed prove the participation of the incident laser in the surface reaction. In order to provide further evidence for the evaluation of the laser light effect, the power-dependent SERS spectra were monitored at 1.1, 2.6, 5.6, and 10.2 mW and are shown in Figure 8b. The SERS spectra recorded at all powers clearly show the peak for the trans isomer at 1444 cm−1, but the peak at 1515 cm−1 due to the cis isomer was almost negligible at 1.1 mW. The power-dependent SERS spectra thus clearly show the participation of the incident laser in the surface-catalyzed reaction. To study the effect of localized surface plasmon resonance (LSPR), the substrate was altered from Ag NPs to Au NPs, but adsorption of HAA was not observed under similar experimental conditions. This further proves the fact that LSPR indeed plays a key role in surface-catalyzed oxidation of HAA and its isomerization. The surface-catalyzed oxidation of HAA was also monitored using the TDUV−vis absorption spectra. It should be emphasized here that the silver colloid containing HAA was
much faster in O2-saturated solution (Figure 7b) than in aerated condition (Figure 1a) or in N2-bubbled solution (Figure 7a). These results show that O2 indeed plays a key role in the isomerization of DHDCAB on the surface of Ag NPs. The differences in the intensities of cis and trans isomers can be explained as follows: It is known that the surface-plasmonassisted catalytic reactions on the surface of Ag NPs occur via the participation of electrons.29 At acidic pH, a two step reaction of O2 with thermal electrons and H+ results in the formation of HO2 that has pKa of 4.8 (eqs 1 and 2). O2 + e− → O2− O2− + H+ → HO2
(1)
(pK a 4.8)
(2)
O2−
Thus, at acidic pH, the relative concentration of is low as compared to that at pH ≥ 7.8. As observed from the TDSERS spectra, at acidic pH, the change in environment (air, N2, O2, N2O) has less significant effect on the intensity of the cis isomer of DHDCAB. In contrast, at pH 10, ionization of carboxylate group and partial deprotonation of phenolic OH occurs that leads to an increase in the overall negative charge within the molecule. To minimize the electrostatic repulsion in DHDCAB, the molecule probably stabilizes itself by attaining the trans conformation. In aerated solution at pH ≥ 7.8, O2− formation results in the trans to cis isomerization within 24 h. In N2bubbled solution, the isomerization is slower (48 h) than in air, and this may be due to the leakage of O2. In O2-saturated solution, the isomerization is very fast (2 h). Thus, O2 indeed plays a key role in the surface-catalyzed trans−cis isomerization reaction at pH ≥ 7.8. The surface-catalyzed oxidation of HAA to form the trans and cis isomers of DHDCAB under different conditions is shown in Scheme 1. Scheme 1. Schematic Representation for the Formation of Azo Derivative of HAA
It can thus be concluded that the relative intensities of trans and cis isomers depend not only on the pH of the solution but also on the presence of O2. The effect of O2 on the azo NN str (cis) peak is intriguing. This implies that the trans−cis isomerization of DHDCAB can give an idea about the presence of O2 in solution at pH ≥ 7.8, albeit qualitatively. Moreover, in order to check whether adsorbed reductants like citrate or BH4− on the surface of Ag NPs have any role to play in the surface-catalyzed reaction, we prepared Ag NPs using pulse radiolysis technique without using any stabilizer. The advantage of using ionizing radiation in the reduction of metal ions is that no reductant is added from outside as the NPs get formed by the reduction of Ag+ ions by the radiolytic species (eaq−) of F
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Figure 8. SERS spectrum (1 h) at pH 7.8 recorded at (a) different excitation wavelengths and (b) varying laser power at 514 nm excitation.
observed. The absorption spectrum of pure (10−4 M) HAA is shown in the inset of Figure 9 for comparison. It shows the appearance of two peaks with maxima at 294 and 332 nm. The UV−vis absorption results clearly indicate that the potential energy barrier for the surface-catalyzed oxidation (azo formation) of HAA is relatively low, probably of the order of thermal energy, kT, (0.025 eV). In order to confirm that the formation of the new peak as observed in Figure 9 is not due to aggregation, similar TDUV−vis absorption measurements were performed by adding dilute HCl and 10−1 M NaCl to the Ag colloid. HCl and NaCl are known to initiate aggregation of Ag NPs, and the results are shown in Supporting Information, Figures S3 and S4, respectively. The surface-catalyzed oxidation of HAA was also confirmed when anthranilic acid instead of HAA was added to the Ag NP and the surface-reaction monitored as a function of time. No observable change was found in case of anthranilic acid as seen in the Supporting Information, Figure S5. These observations indeed lead us to conclude that HAA undergoes oxidation to its trans-azo derivative that subsequently isomerizes to the cis form. This was also confirmed using ESI-MS, which showed the m/z peak of parent HAA at 152.04 and for the azo derivative at 297.15. The ESI-MS spectrum is shown in the Supporting Information, Figure S6. The possible mechanism of HAA oxidation over Ag NPs to form DHDCAB is also shown in the Supporting Information, Scheme S1. It is known that aqueous silver clusters (Agn) get oxidized by transferring electrons to suitable acceptors.50 Tripathi has shown the formation of p-benzosemiquinone radical anion using SERS.53 It is suggested that the radical is produced by one-electron reduction of p-benzoquinone on aqueous Ag NPs. The main reason for the formation of radical is attributed to the differences in the redox potential of silver particle and quinone couple. Similar phenomena may be occurring in the present study. The influence of the change in pH on the morphology of Ag NPs, if any, was studied using pH-dependent UV−vis absorption spectrum and is shown in Supporting Information, Figure S7. The results indicate that as the pH of the Ag NP (prepared using citrate reduction) is lowered from pH 6.6 to 3.1, the absorbance at 450 nm gradually goes down. Lowering the pH to 2.8 and further down to pH 2 shows an increase in the absorbance around 700−850 nm, in addition to the decrease in absorbance at 450 nm. These changes in
Figure 9. TDUV−vis absorption spectra of pure Ag NPs (○) and with added 10−4 M HAA at pH 7.8. Inset shows the absorption spectrum of HAA (10−4 M).
not exposed to laser or any other light source prior to absorption measurements. The TDUV−vis absorption spectrum of pure silver colloid (Figure 9) shows a single sharp peak with maximum at 388 nm due to the surface plasmon resonance band.7 On addition of HAA (10−4 M) to the silver colloid the absorption spectrum showed a decrease in the absorbance of the 388 nm band with an additional band appearing at 555 nm. The appearance of the red-shifted peak is attributed to a charge-transfer/aggregate band51,52 induced by adsorption of HAA on the silver surface. Within 10−15 min, the new peak appearing at 555 nm is further red-shifted to 565, 575, and 590 nm, and the appearance of a broad feature with a peak around 451 nm and a shoulder around 367 nm is observed. The formation of this broad feature around 451 nm and the shoulder at 367 nm is attributed to n−π* and π−π* transition corresponding to the trans isomer of DHDCAB. With increasing time, the trans isomer of DHDCAB subsequently isomerizes to the cis form. The absorption spectrum recorded after 24 h of initializing the surfacecatalyzed reaction clearly shows two peaks with maxima at 367 and 466 nm attributed to the π−π* and n−π* transitions corresponding to the cis isomer of DHDCAB. Thus, within 24 h, the isomerization reaction was complete, reaching equilibrium between the trans and cis isomers of DHDCAB. A redshifted peak at 590 nm due to aggregated silver was also G
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ACKNOWLEDGMENTS The authors thank Prof. K. Maiti of TIFR, Mumbai, for providing the Gaussian 03 computational facilities and Dr. D. K. Palit, Head, RPCD, and Dr. B. N. Jagatap, Director, Chemistry Group, for their kind support and encouragement. The authors also thank Shri. P. G. Jaison, FCD, BARC, for his help in recording the ESI-MS spectrum.
absorbance could be due to the aggregation of the particles and/or due to the slight changes in morphology of the Ag NPs. 3 O2 is known to play a vital role in the oxidation reactions of biological molecules.54 The same is true about the pH of the solution as phenols and thiols are highly susceptible to aerial oxidation. It is therefore intriguing to observe the surfacecatalyzed azo formation from HAA and the dependence of trans−cis isomerization on O2 and pH of the solution. This may have important implications as various drugs supported on metal NPs can undergo structural changes. A pH sensor of gold nanorods functionalized with PATP was developed recently55 to measure pH in living cells in order to get information about the surrounding environment. The authors suggested that by measuring the SERS signals of PATP molecules one can get information about the pH as the formation of DMAB depends on the pH of the solution. Though at present it may be a farfetched statement, we hope that a sensor of Ag NPs coated with HAA can be used to detect hypoxia in cells in addition to pH information about the surrounding environment.
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ASSOCIATED CONTENT
S Supporting Information *
TDSERS and TDUV−vis spectra of HAA under different experimental conditions along with ESI-MS spectrum and possible mechanism for HAA oxidation over Ag NPs. This material is available free of charge via the Internet at http:// pubs.acs.org.
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REFERENCES
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4. SUMMARY AND CONCLUSION In this Article, we have demonstrated that the surface-catalyzed oxidation of HAA depends on the molecule’s property. The intramolecular hydrogen bonding in HAA (due to the presence of −OH and NH2 group in ortho position) results in the weakening of the N−H bond leading to the formation of azo derivative of HAA. The surface of support probably plays an insignificant role in the oxidation reaction. Although, thermal energy alone was found to be sufficient for the surface-catalyzed oxidation of HAA and its isomerization, but the dependence of enhanced reaction rates on excitation wavelength and laser power shows that contribution from surface plasmon resonance is not negligible. The thermal electron from the metal surface played a key role in the surface-catalyzed azo formation. The property of the oxidized product was dependent on the external conditions like temperature, pH, and environment (air, N2, O2, or N2O). Here, we have established that the TDSERS technique can serve as an ideal tool to study in situ surfacecatalyzed reactions by uniquely fingerprinting the spectral changes of the surface species. This study clearly points toward a promising new era of surface-catalyzed reactions. It also provides a new strategy not only for the synthesis of azo dyes with a control over the isomerization process by changing the temperature, pH, or environment, but also for distinguishing between the normal and abnormal cells.
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Corresponding Authors
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[email protected]. Phone: +91-22-25590301. Fax: + 91-22-25505151. *E-mail:
[email protected]. Phone: +91-22-25590298. Fax: + 91-22-25505151. Notes
The authors declare no competing financial interest. H
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