pH-Dependent SERS by Semiconductor-Controlled Charge-Transfer

Nov 2, 2012 - (12) Hurst, S. J.; Fry, H. C.; Gosztola, D. J.; Rajh, T. J. Phys. Chem. C. 2011, 115, 620−630. (13) Tarakeshwar, P.; Finkelstein-Shapi...
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pH-Dependent SERS by Semiconductor-Controlled Charge-Transfer Contribution Wei Ji,† Yasutaka Kitahama,‡ Xiaoxia Han,‡ Xiangxin Xue,† Yukihiro Ozaki,*,‡ and Bing Zhao*,† †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan

‡

S Supporting Information *

ABSTRACT: With the explosive development of analysis and detecting techniques based on SERS, the further understanding and exploit of the chemical mechanism becomes particularly important. We investigated the contribution of semiconductor to surface-enhanced Raman scattering (SERS) in metal and semiconductor heterostructure consisting of Ag/ 4-mercaptophenol (MPH)/TiO2. For this, we used the distinctive property, where the band edge position of an oxide semiconductor such as TiO2 is sensitive to the pH value, to control the charge-transfer (CT) contribution. It was found that increasing the pH of the buffer solution negatively shifts the conduction band edge of TiO2, thereby increasing the conductive band electron density at an equilibrium state. Thus, the relative band intensities of Ag/MPH/TiO2 increase in the SERS spectrum, which is attributed to the Herzberg−Teller contribution that occurs via CT. Moreover, because of the slower transport of cations from the pH buffer solution to the surface of TiO2, which results from the space-charge limitation, there is a decay time that is associated with the pH-response process.

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become a mature technology.23−29 To our knowledge, no SERS sensor that is based on the photoelectrical property of a semiconductor combines with a noble metal substrate has been employed for theoretical and practical applications. As we all know, there are two mechanisms that contribute to the huge enhancement.30 The first is electromagnetic enhancement mechanism (EM) that is generated by the resonant excitation of the surface plasmons in metallic particles. The second is chemical enhancement mechanism, which is associated with a photoinduced charge-transfer (CT) transition between the Femi level of substrate and the allowed level of molecule. The CT theory has been well proposed by Lombardi et al.; however, the experimental understanding of the CT contribution to enhancement is still limited due to the lack of appropriate system.30−32 As the explosive development of analysis and detecting techniques based on SERS, the further understanding and exploiting the CT contribution becomes particularly important. Our group has been focusing on the investigation of chemical mechanism using semiconductor and metal-semiconductor heterostructure.33−38 Recently, we found that the motion of a conduction band edge plays an important role in the chemical contribution to SERS.39,40 Laser excitation at 514.5 nm (2.41 eV) is in resonance with the CT transition from Ag/4-mercaptophenol (MPH) to the T2g orbital of TiO2 in an Ag/MPH/TiO2 (the TiO2 film has seven layers)

INTRODUCTION Colloidal semiconductors have been attracting considerable interest as nontoxic and consistent biocompatible materials due to their unique optical and spectroscopic properties.1−4 They are currently being widely applied to various fields, for instance, photoelectrochemical solar cells, lithium batteries, and photodegradation of organic pollutants in water.5−7 Moreover, surface-enhanced Raman scattering (SERS) can be observed for analytes adsorbed on the surface of semiconductor.8−13 SERS is a promising technique that can be used for ultrasensitive chemical detection, biological analysis and diagnosis, and biomedical applications.14−17 A huge enhancement of Raman scattering up to 1014 can be achieved when molecules are adsorbed on or approached to a noble metal surface.18,19 However, generally speaking, noble metals exhibit poor stability and biocompatibility. The appearance of semiconductor active substrates seems to have resolved this problem, but only 103 to 104 enhancement can be achieved by semiconductor active substrates, limiting their potential applications in biological and biomedical analysis and diagnosis. Thus, extensive attempts have been made to achieve higher enhancement, stability, and biocompatibility at the same time. Until now, the fabrication of nonconductor or semiconductor shells has been a widespread strategy to promote biocompatibility and protect SERS marker molecules, which means that only the noncore properties of semiconductor materials have been utilized.20−22 It is commonly known that colloidal semiconductors exhibit excellent optical and electrical properties and that semiconductor-based photovoltaic sensors have © 2012 American Chemical Society

Received: September 5, 2012 Revised: November 2, 2012 Published: November 2, 2012 24829

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(CCD) (Princeton Instruments) was used. Laser radiation with the wavelengths of 514.5 nm from an Ar laser (Spectra Physics) was used.

nanosystem. According to CT theory, the variations of conduction band position will certainly influence the degree of CT in such a system. Consequently, increasing the degree of CT represents an increase in the relative band intensity in the SERS spectrum of the MPH. Thus, it is possible to realize such SERS sensor in theory by combining the SERS chemical mechanism and semiconductor photovoltaic property. Obviously, poor reproducibility of the SERS signal, which is ascribed to the irregularity of the substrate, can be avoided through the chemical enhancement. However, the lack of a systematic study on the semiconductor contribution to SERS has long hampered the development of such sensor. Therefore, appropriate new experimental probes and theoretical models should be investigated to realize this type of advancement. Herein, we propose a pH-dependent SERS enhancement experiment, which is based on the special photoelectrical property of the band edge position of oxide semiconductors, such as TiO2 is sensitive to the pH value. It is clearly different from previous approaches, which utilized variations in molecular protonated and deprotonated forms as pH values under ambient conditions.41−43 In this work, the observed enhancement of nontotally vibrational modes is correlated with the CT mechanism due to the variation in the conduction band of TiO2. The system described here could be used to assist us in understanding the CT mechanism of SERS, thereby providing a basic method for carrying out investigations using the SERS mechanism. Moreover, this work also opens up a novel and effective idea for designing biocompatibility nanosensors for intracellular applications.

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RESULTS AND DISCUSSION The heterostructure that we developed is illustrated in Figure 1. To construct this device, a SSG process, which is based on a

Figure 1. Schematic diagram of the heterostructure. A colloid TiO2 film was prepared using a layer-by-layer self-assembly approach. In this work, the TiO2 film has seven layers.

layer-by-layer self-assembly approach, was used.39,46 The Ag nanoparticles and TiO2 film had diameters of about 50 and 5.6 nm, respectively. Before acquiring a Raman spectrum of the MPH molecule at each pH, the substrate was immersed in a citric acid/disodium hydrogen phosphate buffer solution on which a 514.5 nm laser was already focused. The SERS spectrum was collected immediately when the laser illuminated the substrate. Figure 2a,b shows representative SERS trajectories obtained by immersing the substrate in a pH 2.5 buffer solution and by exposing it to air. Most notably, as the exposure time increased, the intensities of the bands at 1572, 1479, 1291, 1163, 899, and 701 cm−1 decreased (Figure 2a). In contrast, the intensities of the bands at 1477, 1072, 1007, 825, and 635 cm−1 remained constant. The former bands were assigned to b2 modes, whereas the latter bands were associated with normal modes of a1 symmetry.39 However, no such trends were observed when the nanosensor was exposed to air (Figure 2b). We selected two representative bands, a typical a1-type band at ∼1072 cm−1 (1), and a b2 band at ∼1163 cm−1 (9b), for detailed analysis of the variation tendencies because they were fairly intense and relatively isolated from nearby bands. Figure 3 shows the variations in intensities of the two bands at 1072 and 1163 cm−1 and the corresponding relative band intensity of MPH in the Ag/MPH/TiO2 nanosystem. The intensities decreased as the exposure time increased for both situations: in air and in the buffer solution. The decrease in the intensity may have been due to the laser heat effect. Prolonged exposure of substrates to a laser may destroy the nanostructure of Ag nanoparticles or induce desorption of the adsorbate47 and therefore result in the loss of SERS intensity. As can be seen in Figure 3b, the SERS relative band intensity was negligibly different when the substrate was exposed to air. In the pH 2.5 buffer solution, nevertheless, it underwent pronounced loss as the exposure time increased and finally achieved an equilibrium value of ∼0.4 after 100 s of illumination (Figure 3d). The intensity ratio reached an equilibrium state after 100 s of laser illumination, after which further SERS spectra were measured in the range 100−150 s. Figure 4a shows the equilibrium state SERS spectra of MPH; it was found that significant changes in the relative band intensities occurred

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EXPERIMENTAL SECTION Materials. Poly(diallyldimethylammonium chloride) (PDDA): Mw = 200 000−350 000, 20 wt % aqueous solution) and MPH were obtained from Sigma. All other chemicals were analytic grade and were acquired from Beijing Chemical Reagent Factory and used without further purification. Preparation of Silver Films. The Ag colloid was prepared using a conventional synthetic route that has been reported elsewhere.44 In brief, 200 mL of a 1.0 mM aqueous silver nitrate solution was added to 250 mL three-necked bottles and then heated to 85 °C with rapid stirring under reflux. A 4 mL solution of 1% trisodium citrate was added to the solution, after which it was boiled for 40 min. Glass slides were immersed in a boiling solution prepared by mixing 30% H2O2 and 98% H2SO4 with a volume ratio of 3:7. After cooling, the glass slides were rinsed repeatedly with water. As a result, the glass surface was covered with hydroxyl. They were then immersed in a 0.5% PDDA solution for 1 h. After exhaustive rinsing by water and drying by nitrogen, the PDDA-coated slides were soaked in silver colloid for 6 h and then rinsed with water.45 Preparation of TiO2 Layer. Assembled Ag nanoparticles were coated with a TiO2 layer using the surface sol−gel (SSG) process originally developed by Ichinose et al.39,46 In brief, glass slides covered with a layer of Ag nanoparticles were immersed in 4-MPH solution (10−3 M) for 1 h. After modification with 4MPH molecules, the slides were transferred into titanium(IV) butoxide (100 mM in a 1:1 (v/v) mixture of toluene/ methanol) for 8 min and then washed with methanol. Finally, the slides were submerged in water for 1 min and then dried with nitrogen gas and thus completing one adsorption cycle. Additional TiO2 layers were fabricated by repeating the cycle. Instrumentation. A Raman spectrophotometer (model RS2100, Photon Design) equipped with a charge-coupled device 24830

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Figure 2. Representative trajectories of SERS spectra of MPH in the Ag/MPH/TiO2 nanosystem, illuminated with a laser power of 15 mW with an average spot size of 1 um in diameter. The integration time for each spectrum was 0.5 s. The trajectories were obtained (a) by immersing the substrate in a pH 2.5 buffer solution and (b) by exposing the substrate to air. In each panel, SERS spectra collected at 1 (cyan curve), 5 (violet curve), and 200 (green curve) s after starting the laser illumination are also plotted. For both trajectories, intensities are normalized using the band at 1072 cm−1 (1a1). Assignments of Raman bands are given on the tops of the trajectories.

Figure 3. Variations in band intensities of a typical b2-type band at 1162 cm−1 (9b) and a typical a1-type band at 1072 cm−1 (1a1) in a SERS spectrum of MPH as a function of illumination time. The decay profiles were obtained by (a) exposing the substrate to air and by (c) immersing the substrate in a pH 2.5 buffer solution. The plots in panels b and d are the corresponding relative intensities of the 1162 (9b) and 1072 cm−1 (1a1) bands. The marked decay of this ratio mainly reflects a large intensity change in the 1163 cm−1 (9b) band.

lost their intensities as a function of the illumination time. The relative band intensity, moreover, decreased and therefore reached equilibrium, indicating that there was a decay time associated with this process. Considering that similar decay profiles are commonly observed in the photocatalysis degradation of organic molecule experiments,51−53 the pHdependent SERS behavior of the Ag/MPH/TiO2 nanosystem was further evaluated by measuring the repetitive cycling of

when the substrate was immersed in different pH buffers. The relative band intensities as a function of surrounding pH are plotted in Figure 4b. Obvious fluctuations can be observed in the relative SERS intensities of the bands at 1163 and 1072 cm−1. In general, we considered these phenomena, especially the changes in the relative Raman band intensities between different vibrational modes, to be closely related to the chemical mechanism of SERS.48−50 However, as can be seen in the profile of Figure 3d, both the b2 and a1 vibrational mode bands 24831

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Figure 4. (a) Equilibrium state SERS spectra of MPH in the Ag/MPH/TiO2 nanosystem measured in buffer solutions with different pH using 514.5 nm excitation (laser power is 15 mW). All spectra were measured with the same acquisition time at 150 s after starting the laser illumination. (b) Integrated relative intensities of the 1162 (9b) and 1072 cm−1 (1a1) bands for pH ranging from 2 to 9. The error bars indicate the equilibrium signal fluctuations for each pH buffer solution.

Figure 5. (a) Recovery trajectories of the relative band intensities of the 1162 (9b) and 1072 cm−1 (1a1) bands in the same point on the substrate, which reached equilibrium state in a pH 2.5 buffer solution. Sample spectra were taken from repeated SERS spectra of MPH at the same point in the Ag/MPH/TiO2 nanosystem (b) when exposed to air and (c) when immersed in the pH 2.5 buffer solution. (d) Relative band intensies of 1162 (9b) and 1072 cm−1 (1a1) bands for MPH measured following repeated changes upon exposure to ambient air and the pH 2.5 buffer solution. Note that the SERS signals were irreversibly lost after several cycles. (The spectra were obtained with a laser power of 15mW.)

relative band intensities and by changing the laser power under ambient conditions between the air and pH buffer solution. If b2-type bands in the SERS of Ag/MPH/TiO2 nanosystem stemmed from a photocatalysis degradation product, then a nonreversible signal and intimate relationship with the laser power would have been observed. Taking into account the fact that the SERS intensity decreased to a great extent with prolonged irradiation, an indispensable test condition should be considered first. As shown in Figure 3, the relative band intensities of b2 and a1 vibrational modes decreased to their equilibrium values after 100 s of illumination under ambient conditions in pH 2.5 buffer solution. The end result was that the ratio was fairly invariant with further extended illumination time. Thus, the substrate was illuminated for 100 s by the laser

before SERS data were collected for the pH 2.5 buffer solution. However, the relative band intensities were recovered immediately (Figure 5a) when the same point just detected in the pH 2.5 buffer solution was selected as the study object. Therefore, only 1 s of illumination was needed before collecting the SERS data for exposure to air. Figure 5b,c shows SERS spectra of Ag/MPH/TiO2 measured under exposure to air situation and pH 2.5 buffer solution with a laser power of 15 mW. It should be noted that the b2-type band intensities significantly decreased relative to a1-type band intensities for the pH buffer solution, whereas they recovered again upon exposure to air. There were barely any different in the spectral patterns between the spectra shown in Figure 5b,c, except for the relative band intensities. It is very likely that the 24832

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Figure 6. Laser power dependence of the relative intensities of the 1162 (9b) and 1072 cm−1 (1a1) bands of MPH in Ag/MPH/TiO2 as a function of illumination time, obtained in (a) air and in (b) pH 2.5 buffer solution. (c) Statistical analysis of changes in integrated relative intensities for various situations. The error bars indicate the equilibrium signal fluctuation under each ambient condition.

mentioned above, we attributed these findings to CT interactions rather than to the catalysis in this system. Thus, the question arises as to which of these processes was responsible for the experimental observations related to the decay time and pH dependent SERS. Normal Raman and SERS spectra of MPH are also affected by the pH of the solution. pH-dependent changes involve variations in the ring modes at 392 (7a), 636 (12), and 1003 (18a) cm−1, which are attributed to the deprotonation of the MPH hydroxyl group.55 However, the hydroxyl group already reacted with TiO2 to form the Ti−O bond in our system. This means that the direct molecular interaction with the pH buffer solution should be excluded. If the Ti−O bond breaks in pH 2.5 buffer solution, then the intensities of b2 mode band cannot recover again. Thus, the SERS spectrum variations shown in Figure 2b must therefore be caused by the interaction of the pH buffer solution with TiO2. Our previous work has shown that the conduction band edge motion of TiO2 plays a major role in the relative band intensities of SERS.40 Nontotally symmetric vibration modes coupled to the CT transition are selectively enhanced when the excitation wavelength exceeds the optical absorption threshold of the CT complex. The CT contribution reaches the maximum in resonance when the CT excitation energy matches the incident laser energy. Here the band edge position of TiO2 is sensitive to the surrounding pH. The relationship between the conduction band edge level (ECB) and the pH is given as follows:56

changes in the relative band intensities were due to the chemical mechanism. Moreover, the relative band intensities were fully restored for the buffer solution. However, they could not be restored to their original values upon exposure to air. At present, these reversible SERS spectra strongly suggest that the appearance of the pH-dependent SERS spectra had nothing to do with the photoinduced degradation of MPH molecules. The independence of the relative band intensities of b2 and a1 vibrational modes relative to the laser power as shown in Figure 6a is further evidence that the pH dependent SERS was extraneous to catalysis degradation. As can be seen in Figure 6b, the relative band intensities decrease to a constant equilibrium value, indicating that the equilibrium value was fairly invariant with respect to the laser power. However, increasing the laser power reduced the time to achieve equilibrium (Figure 6b), which means that the laser power affected the decay time. In general, in a photoinduced CT process, increasing the power of the laser increases the number of photons in the light beam and subsequently increases the number of excited electrons.54 This means that as expected the transient time is shortened with higher laser power. However, the energy of each emitted electron does not depend on the power of the laser; it depends only on the energy of the frequency of individual photons that are present in the laser bean. Therefore, the laser power only influences the time that is required to achieve equilibrium rather than the equilibrium value for the buffer solution. On the basis of this consideration and the analysis of the affected parameters, including the laser power and the reproducibility of SERS signals, the above results indicate the following. First, a pH-dependent SERS spectrum for the Ag/ MPH/TiO2 nanosystem measured at the same point is reversible under ambient conditions in air and in buffer solution. Second, the laser power merely influences the time that the system requires to reach equilibrium rather than the equilibrium ratio of the b2 and a1 vibrational modes. As

ECB = constant − 0.059pH

(1)

It can be seen that the ECB usually shifts negatively by ∼0.059 eV per pH unit (in aqueous solution at room temperature). According to our previous work the negative shifts in the potential of TiO2 indicate that Ag/MPH/TiO2 would turn off resonance, and thus the relative band intensities decrease 24833

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Figure 7. (a) Schematic energy level diagram for the Ag/MPH/TiO2 nanosystem and buffer solution interface. The ECB of TiO2 shifted negatively with increased pH of the buffer solution, and thus the corresponding conductive band electron density increased when using the same laser energy. (b) Schematic diagram of forming the compensating space charge layer. As the electrons continuously reacted with the H+ ions, a positive charge layer was created at the interface between TiO2 and the bulk solution. This layer retarded retards the transport diffusion of H+ ions from the bulk solution to the surface of TiO2.

following the CT mechanism of SERS.40 In addition, MPH molecules are absorbed onto the surface TiO2 in the Ag/MPH/ TiO2 nanosystem. Such types of anchoring groups attached to TiO2 do not change the dependence of the ECB on the pH. Rather, they affect the density of the electronic states and thus interfacial CT rates.57 Because the occupation of the bands obeys Fermi-Dirac statistics, the conductive band electron density at equilibrium can be written as follows:56,58 ne = Nc exp[− (ECB − Ef )/kT ]

electrons continuously reacted with the H+ ions, a compensating space-charge layer must have been created at the interface between TiO2 and the bulk solution as well between the Ag and the bulk solution. (See Figure 7b.) Apparently, the compensating space-charge layer retarded the charge that was compensated by ions diffusion to or from the TiO2−buffer solution interface, Ag-buffer solution interface, or both.61 Therefore, a long decay time occurred for the pH buffer solution corresponding to the change in the ECB of TiO2. From the above, we note that the Ag/MPH/TiO2 nanosystem involving two CT processes, including a fast photoinduced CT into TiO2, is independent of pH. Another slower change in the ECB was present and distinctly different from that prior to light absorption. Hence, the variation shown in Figure 3 consisted of a fast initial phase and then a slow decline.

(2)

where Nc is the accessible density of states in a semiconductor conduction band, ECB and Ef represent the conduction band and the Fermi level, respectively, k is Boltzmann’s constant, and T is the absolute temperature. Because the semiconductor Fermi level lies between the conduction band and the valence band level, the term −(ECB − Ef) is less than zero. Clearly, increasing the pH of the buffer solution negatively shifts the ECB of TiO2 and thus increases the conductive band electron density at equilibrium state. (See Figure 7a.) This variation reflects the increase in the relative band intensities in the SERS spectrum due to the Herzberg−Teller contribution via CT.48 For a quantitative study of the relative CT contribution to the SERS intensity, the degree of CT was also calculated (Figure 4b). It exhibited the same trend as the conductive band electron density. All relative band intensities started to decay from the initial value of ∼1.3 for the pH buffer solution. The same ratio was also observed for exposure to air, whereas it only fluctuated instead of decaying with time. According to the previous study, the relative band intensity of ∼1.3 was assigned to the photoinduced transfer of electrons from Ag nanoparticles to the conduction band of TiO2.39 Such photoinduced CT generally occurs on a femto- to picosecond time scale and reaches equilibrium state immediately, and thus the relative band intensity in the SERS spectrum instantaneously reached a maximum of 1.3 and then started to decay. However, the pHdependent ECB of colloid TiO2 is a slow kinetic process.59 The mechanism of pH-dependent ECB involved a reaction between electrons and H+ ions at the TiO2 surface.29,60 Therefore, the local change in the H+ concentration at the TiO2 surface was significantly greater than that in the bulk solution. As the

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CONCLUSIONS In conclusion, we reported a pH-dependent SERS based on the distinctive photoelectrical property of a semiconductor that is sensitive to pH. We found that the SERS signals of Ag/MPH/ TiO2 vary with the pH of the surrounding environment. Several parameters were considered in our investigation. First, the pHdependent SERS spectrum of the Ag/MPH/TiO2 nanosystem measured at the same point is reversible in ambient air and in buffer solution. Second, the laser power does not change the equilibrium ratio of the b2 and a1 vibrational modes. Rather, it influences the time that is needed for the nanosensor to reach equilibrium. Third, increasing the pH negatively shifts the conduction band edge of TiO2, and thus the conductive band electron density at equilibrium increases. Consequently, the relative band intensities of MPH molecule that increase in the SERS spectrum can be attributed to the Herzberg−Teller contribution via CT. Moreover, two different models are involved in the CT process. One is photoinduced CT from Ag nanoparticles to the conduction band of TiO2, which results in the relative band intensity reaching equilibrium immediately. Another is related to the reaction of electrons with H+ in the solution. Because of the slower transport of cations from the pH buffer solution into the surface of TiO2, which results from the space charge limitation, a decay time exists for the pH response process. Overall, these observations strongly suggest 24834

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that a SERS nanosensor can be constructed using metal− molecule−semiconductor heterostructure, which is based on semiconductor characteristics.

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ASSOCIATED CONTENT

S Supporting Information *

Sample spectrum taken form the SERS trajectory for the variation profiles of 1162 (9b) and 1072 cm−1 (1a1) bands and SERS spectrum obtained for exposure to air at different laser power. Calculation of the degree of charge transfer. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +86 431 8519 3421; Tel: +86 431 8516 8473 (B.Z.). E-mail: [email protected]; Fax: +81 79 565 9077; Tel: +81 79 565 8349 (Y.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation (20921003, 20973074) of P. R. China; Specialized Research Fund for the Doctoral Program of Higher Education (20110061110017); the 111 project (B06009); and the Development Program of the Science and Technology of Jilin Province (20110338).

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