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Ag2O/TiO2 Nanocomposite Heterostructure as a Dual Functional Semiconducting Substrate for SERS/SEIRAS Application Chen Tan, Zhiyun Zhang, Yanqi Qu, and Lili He Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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Ag2O/TiO2 Nanocomposite Heterostructure as a Dual Functional Semiconducting Substrate for SERS/SEIRAS Application Chen Tan, Zhiyun Zhang, Yanqi Qu, Lili He* Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA. *E-mail: [email protected]

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ABSTRACT Surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRAS) are complementary and powerful techniques for molecular characterization and detection. However, studies on substrates that can enhance both Raman and IR singles are extremely scanty. Here we reported a hybrid semiconductor material (Ag2O/TiO2) coupled with a portable solid support served as a dual functional platform for both SERS and SEIRAS applications. A facile two-step deposition method was used to synthesize Ag2O/TiO2 nanocomposite on a flexible polymeric membrane without bringing any external chemical capping agent and background signal. The presence of Ag2O was proposed to enrich the photogenerated electrons onto TiO2 surface and facilitate the photon-induced charge transfer (PICT) between TiO2 and adsorbate. The heterostructure of Ag2O/TiO2 could bring additional enhancement. The enhancement factor from such hybrid semiconducting substrate was at least one or two orders of magnitude over traditional semiconducting materials and comparable to noble metals. Additionally, this substrate enabled the ultra-trace detection regardless of the more Raman- or IR-active molecules and displayed distinct quantitative capacities for SERS and SEIRAS. High reproducibility of the SERS/SEIRAS spectra further confirmed the reliability and reproducibility of our substrates. KEYWORDS: semiconducting substrate, Ag2O/TiO2, SERS, SEIRAS, quantitative capacity, reproducibility


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INTRODUCTION Owing to the richness of molecular information, surface selectivity and sensitivity, surface enhanced Raman spectroscopy (SERS) has emerged as a highly powerful tool for molecular detection and characterization in various disciplines.1,2 Surface enhanced infrared absorption spectroscopy (SEIRAS) is a complementary technique to SERS. A combination of SERS and SEIRAS will provide the greatest analytical capabilities from a practical perspective.3,4 However, SEIRAS has been much less developed and applied as compared to SERS. The major challenge for preventing SEIRAS to be as popular as SERS is the lack of effective SEIRAS substrates. Apart from the noble metals, it has been previously observed that the semimetals, semiconductors and polar dielectric nanostructures also play important roles in the SEIRAS enhancing capacity.5,6 Nevertheless, it has to be mentioned that whatever the types of SEIRAS substrates, the SEIRAS-active metal/nonmetal films should be fabricated on a specific IR support for operation, i.e. highly IR refractive (Si, Ge, and ZnSe) and transparent (CaF2 and BaF2) materials. Up to now, the most commonly preparation methods for SEIRAS-active film on IR supporting substrate are high vacuum evaporation7,8 and chemical deposition.9,10 Unfortunately, both techniques have some problems and are far from robust. For instance, the vacuum evaporated film always shows weak film adhesion, low reproducibility and high cost. Although the chemical deposition technique increases the uniformity of substrate, it is plagued with issues of demand for sophisticated and time-consuming procedure. Furthermore, difficulty in removing chemisorbed metal film from the support base greatly restricted the efficiency for large sample. In view of these, there is a critical need for a cost-effective, easily-prepared and stronger alternative substrate for SEIRAS. The objective of this study is to develop a dual functional substrate that can be practically used


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for both SERS and SEIRAS measurement. The key of this development is to build a SERS/SEIRAS-active structure on a portable solid support. Semiconductor nanostructures have been demonstrated to be easily fabricated with high reproducibility and stability in comparison to noble metal nanostructured substrates.11,6 TiO2, a traditional n-type semiconductor, has attracted considerable attention in the catalytic applications due to its ultraviolet light photoactivation. TiO2-based SERS substrates offer not only a biocompatibility, but also a greater chemical and mechanical stability. Similar to other semiconductors, however, TiO2 exhibits low enhancement factor (EF) of SERS as compared to noble metals, usually in the range of 10-102. The absence of surface plasmon resonance (SPR), electron-hole recombination and low interfacial charge transfer rates are responsible for the weak SERS effect. To overcome this problem, researchers developed the TiO2 based heterojunction such as deposited semiconductor couple with TiO2, which could reduce the recombination of electrons and holes, and effectively improve photocatalytic efficiency. For this purpose, Ag and Ag based oxides are the most suitable for industrial application owing to their high efficiency, low cost and easy preparation. It was reported that Ag-TiO2 hybrid nanocomposite not only exhibited satisfied photocatalytic capacity,12,13 but also served as a promising SERS-active substrate.14 More interestingly, Ag oxides (mainly Ag2O), a p-type semiconductor, was found to act as a more superior photocatalyst than Ag by generating p-n nanoheterojunction with TiO2.15 However, the synergetic effect of Ag2O and TiO2 to enhance SERS signals has not been yet investigated. Furthermore, the SPR frequency of TiO2 and Ag2O located in the infrared region would endow them to be effective candidate in SEIRAS substrates. Thus, it is worthwhile to tailor-make Ag2O/TiO2 hybrid nanostructured substrate for SERS and SEIRAS simultaneously. Herein, we develop a specifically designed and dual functional three-dimensional (3D)


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semiconducting substrate that simultaneously enhances Raman scattering and infrared vibrational absorption. Our strategy is based on a two-step deposition of Ag2O/TiO2 nanocomposite on a flexible polymeric membrane. This deposition technique does not bring any external chemical capping agent and yielded singles with a high degree of reproducibility and no background signal. The synergetic contribution of Ag2O and TiO2 nanoparticles can significantly improve the interfacial charge transfer between the molecules and substrate. It is also worth noting that a kind of cheap and stable polymeric membrane, i.e. polyvinylidene fluoride (PVDF) is selected as a portable support for Ag2O/TiO2 nanocomposites. The integration of a flexible polymer membrane provides a unique flexibility and strength. Therefore, this substrate can be used for interior FTIR-attenuated total reflection (FTIR-ATR) mode, where external pressure is required to ensure good contact between the surface species and ATR crystal. Additionally, the detachability of the membrane suggests that our substrates can be truly effectively and practically applied in the Raman and IR instrument for real analytical applications. To our knowledge, it is the first attempt to apply the hybrid semiconductor material (Ag2O/TiO2) coupled with a portable membrane support as a dual functional substrate for SERS and SEIRAS simultaneously. EXPERIMENTAL METHODS Preparation of Substrate. TiO2 stock suspension was prepared by suspending an accurate amount of TiO2 nanopowders (20 nm, anatase) in deionized water and the final concentration was 1 g/L. The suspension was vortexed vigorously for 2 min at ambient temperature, and then submitted to a sonication process for 5 min. The obtained stock suspension was kept at 4 oC in the dark. Before every use, the stock suspension should follow the above pretreatment to ensure the homogeneous distribution of particles. The substrate was prepared as follows: 1 mL of TiO2


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suspension was filtered through a Durapore ® (made of polyvinylidene difluoride, PVDF) membrane filter (220 nm pore size, 13 mm OD) using a syringe filter holder (Sartorius Stedim Biotech Gmbh, Germany). The filtration was repeated two times. Due to the significant aggregation TiO2, we can assume that most of the TiO2 particles can be trapped on the membrane. The deposition of TiO2 can shield the Raman/IR signal of PVDF membrane because TiO2 has no absorbance in the range of 4000-1000 cm-1. TiO2 film also provides a smooth and flat surface for analytes adsorption. The membrane was then removed from the filtration apparatus and washed with deionized water. Afterwards, the as-prepared TiO2-deposited PVDF membrane was immersed into a 10-3 M AgNO3 solution in a 12-well plates placed in a dark room. The solution was irradiated for 0.5 h by a 40 W mercury lamp with a maximum emission at 254.6 nm, and the UV lamp to the substrate distance was kept at 15 cm. The deposition of Ag element on TiO2 surface was triggered via a photoreduction process under UV irradiation. After illumination, the TiO2 surface became dark brown. The obtained Ag modified TiO2-deposited membrane was thoroughly washed with distilled water and dried at room temperature in the dark. The loading of Ag was estimated by measuring the unused precursor concentration after the photodeposition using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500ce, Santa Clara, CA). General Characterization. The membrane was vacuum-dried overnight and characterized by scanning electron microscopy (SEM) using a FEI Magellan 400 (FEI, OR) with an accelerating voltage of 5 kV under low vacuum conditions. The Ag2O/TiO2 structure was evaluated by high resolution transmission electron microscopy (HRTEM, JEOL, 200FX, USA) coupled with energy dispersive spectrometry (EDS). X-ray photoelectron spectroscopy (XPS) studies were carried on a Physical Electronics Quantum 2000 spectrometer using a monochromatic Al Kα


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excitation at a spot size of 200 µm with pass energy of 46.95 eV at 15o take-off angle. All banding energies were referenced at 284.6 eV, as determined by the location of the peak C 1s spectra, which gave banding energies values within an accuracy of ± 0.1 eV. X-ray powder diffraction (XRD) patterns of Ag2O/TiO2 were obtained using a PANalytical X'Pert diffractometer using Cu Kα radiation. UV-vis diffuse reflectance spectra (DRS) were recorded on a LAMBDATM 1050 UV/vis/NIR spectrometer along with 150-mm integrating sphere (PerkinElmer, Inc., Shelton, CT USA). SERS/SEIRAS Measurements. In this study, 4-mercaptobenzoic acid (4-MBA) and bovine serum albumin (BSA) were chosen as model analytes to investigate the performance of the asprepared substrate for SERS/SEIRAS detection. The Raman and IR samples were prepared as follows: the substrate was soaked in 1 mL analytes solution of different concentrations in a 12 well plate. After 1 h incubation, the substrate was taken out from plate following by washing with deionized water and air-dried. For reference, the bare TiO2-deposited PVDF membrane without Ag deposition was incubated with analytes solution. For SERS measurement, the dried substrate was directly placed on the stage for scanning. Raman spectra were obtained using a DXR Raman microscope (Thermo Fisher Scientific, Waltham, MA) equipped with a 780 nm excitation laser and a 10 × objective. The resulting laser spot diameter was about 3.0 µm with a spectral resolution of 5 cm-1. The Raman measurements were performed with 5 mW and 50 µm slit aperture for 2 s integration time. More than ten discrete locations were randomly chosen on substrate under Raman microscope and analyzed within a spectrum range of 100-3000 cm-1. The attenuated total reflectance (ATR)-FTIR spectra were collected by IRTracer-100 Shimadzu equipped with a Pike MIRacle ATR accessory and a high pressure clamp. The substrate was turned around to closely contact with ATR crystal upon pressure. Four different positions on


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each substrate were scanned from 500 to 4000 cm-1. Each experiment was carried out in triplicate. For the mapping measurement of SEIRAS, a Shimadzu AIM 8800 microscope with an auto-XY stage positioning was used. FTIR spectrometer and a 6464 MCT FPA detector (Stingray imaging Spectrometer) was used to acquire data at 8 cm-1 resolution under N2 purge. This allowed information on analyte distributions to be obtained from a 450 µm × 450 µm region with a step size of 50 µm. Therefore, both the scanned area for SERS and SEIRAS contain 10×10 individual points which should be statistically presentative for the sample. Imaging data were analyzed with IR solution software. The second deviation of peak intensity were calculated for each spectrum, and infrared images were created on the basis of these second derivative of IR absorbance. Data Analysis. All spectra were analyzed using OMINICS software (Thermo Fisher Scientific, Waltham, MA). The spectra from different locations in each sample were averaged to get a final spectrum. Data are presented as a mean value with its standard deviation indicated (mean ± SD). Raman enhancement factor (EF) for our fabricated substrate was calculated using the following equation: =

/

/

Where ISERS and IRS represent the SERS peak intensity of the 10 mg/L 4-MBA (BSA) at 1582 cm-1 (1449 cm-1) on Ag2O/TiO2 and the Raman peak intensity of 10 g/L 4-MBA (BSA) at 1582 cm-1 (1449 cm-1) on the pure TiO2. CSERS and CRS represent the concentration of 4-MBA (BSA) incubated with Ag2O/TiO2 and pure TiO2, respectively. IR enhancement factor (EF) for our fabricated substrate was calculated using the following equation:


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=

/

/

Where ISEIRAS and IIRAS represent the SEIRAS peak absorption of the 10 mg/L 4-MBA (BSA) at 1590 cm-1 (1538 cm-1) on Ag2O/TiO2 and the IR absorption of 0.1 g/L 4-MBA (BSA) at 1590 cm-1 (1538 cm-1) on the bare TiO2. CSEIRAS and CIRAS represent the concentration of 4-MBA (BSA) incubated with Ag2O/TiO2 and pure TiO2, respectively. To determine EF value, we assumed that all of the 4-MBA molecules and BSA were adsorbed on the silver surface. Thus, the EF value we obtained was an underestimated value, and the real EF value could actually be some orders of magnitude higher. The standard deviation in SERS and SEIRAS enhancement σ for the substrate was estimated as follows:16 1 1 = − ̅ × 100% , ̅

!

̅ =

1

!

Where N is the number of SERS/SEIRAS spectra of analytes, which are measured at different locations on the same substrate, Ii is the analytes Raman or IR signal detected at the ith location,

and ̅ is the average signal intensity. Herein, a total of 100 (10 × 10) SERS/SEIRAS spectra have

been measured.

RESULTS AND DISCUSSION Substrate Fabrication and Characterization. The facile two-step deposition technique was illustrated schematically in Figure 1 (details of experimental procedures are provided in the experimental section). Appropriate volume of TiO2 suspension (particle size 20 nm) was filtered through a PVDF membrane (220 nm pore size) using an injection syringe. Due to the significant aggregation of TiO2, we assumed that most of the TiO2 particles were trapped on the membrane.


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This was also confirmed by the transparent solution after filtration (Figure 1a). Different from the calcined TiO2 film usually used for Ag nanoparticles growth, we directly deposited TiO2 on/in PVDF membrane to form a porous net structure. The 3D nanostructures not only possess high surface-to-volume ratio for analytes adsorption,17 but also facilitate the formation of Ag (I) oxides (Ag2O) onto TiO2 surface forming heterostructure.15 On the other hand, it is important to point out that PVDF membrane itself can produce Raman and IR singles, especially the strong IR adsorption in the range of 800-1500 cm-1 (Figure 1b and c). To shield the signal from PVDF membrane, the concentration of TiO2 was optimized to completely cover the membrane, as TiO2 did not have any IR absorption in the mid-infrared range. It was noted that after deposition of TiO2 nanoparticles at 1 g/L on PVDF membrane, no observable IR signals can be detected ranging from 1000-4000 cm-1 (Figure 1b and c). Therefore, the deposited TiO2 film in our study not only served as an active substrate but also provided clean IR background for the ease of signal identification. The subsequent deposition of Ag element on the surface of TiO2 film was triggered in AgNO3 solution via a photoreduction process under UV irradiation. After illumination, the TiO2 surface became dark brown (Insert in Figure 2a and b). It is also noteworthy that no background signal was detected after photodeposition of Ag2O. For Raman measurement the dried substrate was directly placed on the stage, while for FTIR-ATR it was turned around to closely contact with ATR crystal upon pressure.


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Figure 1. Illustration of the two-step deposition technique for fabricating SERS/SEIRAS substrate based on Ag2O/TiO2 nanocomposite film on PVDF membrane. (a) TiO2 suspension before and after filtration. Raman (b) and IR (c) spectra of PVDF membrane before and after TiO2 and Ag2O deposition. The scanning electron microscopy (SEM) images revealed that bare TiO2 surface presented a porous, sponge like network of high roughness and complexity (Figure 2a). Such structure allowed a large contact area between the surface species and substrate, and thus high efficient SERS and SEIRAS. The treatment of UV irradiation in AgNO3 solution induced a slight aggregation of TiO2 nanoparticles and an increase of the surface roughness (Figure 2b). HRTEM and EDS analysis were used to confirm the formation of heterostructure of Ag2O/TiO2 (Figure 2c and Figure S1). A mixture of Ag2O nanoparticles of size 5-10 nm and the relative large TiO2 NPs (~20 nm) were observed. Compared with the precursor TiO2, new XPS peaks of Ag element were found after UV irradiation in addition to the Ti, O, C elements based on XPS survey spectra (Figure S2). The strong XPS peak of Ag 3d at 367.7 eV demonstrated that the predominant silver moiety on the surface of TiO2 was Ag (I) in the form of Ag2O (Figure 2d). The amount of Ag (I) on the surface of TiO2 was ca. 10 atom% (Table S1). The presence of Ag (I) oxides was probably


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due to reverse spillover of oxygen-ions from the TiO2 support onto the surface of metal crystallites.18,19 Using ICP-MS, the loading of Ag was estimated to be 0.7 wt%. The amount of Ag in the substrate can be adjusted by varying the concentration of AgNO3 solution. Here, we used an intermediate concentration 10-3 M based on the observation of a previous report.14 Higher Ag loading may block the light adsorption of TiO2, and thus decrease the overall SERS/SEIRAS activities. This hypothesis needs to be further verified in future experiment. XRD patterns of TiO2 and Ag2O/TiO2 heterostructure were shown in Figure 2e. The strong peak at 2θ=25.5o was representative for (101) anatase phase reflections of TiO2. XRD patterns for Ag2O/TiO2 exhibited peaks corresponding to hexagonal Ag2O {100} and {011} planes at 34.2o and 38.39o, respectively.20

UV-vis diffuse reflectance spectra (UV-vis DRS) further

characterized the composition and microstructures of the substrates (Figure 2f). Bare TiO2 film exhibited a steep adsorption edge located at 380 nm, while Ag2O/TiO2 nanocomposites exhibited an absorption shoulder at around 450 nm indicating surface plasmon absorption.18 Meanwhile, higher photoabsorption was observed in the visible and infrared region (>400 nm). This can be explained by the fact that Ag2O acts as visible-light sensitization with a strong and wide absorption band in the visible/infrared light region.21


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Figure 2. Characterization of Ag2O/TiO2 substrate: SEM images of pure TiO2 (a) and Ag2O/TiO2 nanocomposite (b), insert is the corresponding optical images; HRTEM image of the Ag2O/TiO2 heterestructure (c), XPS spectra of Ag 3d (d); XRD patterns of pure TiO2 and Ag2O/TiO2 nanocomposite (e) and UV-vis DRS of bare TiO2 and Ag2O/TiO2 nanocomposite (f). Raman and IR enhancement. To demonstrate the universality of substrate we have shown here the SERS/SEIRAS spectra of two probe analytes, i.e. 4-mercaptobenzoic acid (4-MBA) and bovine serum albumin (BSA) which have distinct Raman/IR activities. The two analytes can be chemisorbed to the Ag2O/TiO2 surface via covalent interaction between Ag/Ti and their functional groups (-SH, -NH2 or OH). It was shown that the substrate can provide rich and complementary characteristic Raman/IR vibrational modes (Figure 3 and Table S2). In the case of 4-MBA, the ν (C-C) ring-breathing modes (1070 cm-1 and 1575 cm-1) were very strong in the SERS, while the former was not observable in SEIRAS. This phenomenon was also apparent for ν (C=O) at 1284 cm-1. The ν (COO-) at 1400 cm-1 was a good example of a vibration that had strong IR dipole moment, but was SERS inactive due to a small polarizability. Similarly, the


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strong amide II of BSA was detected in SEIRAS, but disappeared in SERS. Furthermore, the characteristic vibrational modes were greatly enhanced and high-quality SERS/SEIRAS spectra of analytes were acquired across much of the Raman/IR fingerprinting region. We observed a Raman enhancement factor (EF) up to 1.8×105 (4-MBA) and 1.1×104 (BSA) on Ag2O/TiO2 nanocomposite film (see details on EF calculation in data analysis). The remarkable Raman enhancement factor of 4-MBA from our Ag2O/TiO2 substrate was at least two orders of magnitude higher than that from Ag-TiO2 nanocomposites reported from another group.14 Meanwhile, the IR enhancement was also found to be as high as 45.8 (4-MBA) and 55.6 (BSA), respectively. We speculated that such superior SERS/SEIRAS activities are attributed to the synergetic contribution of Ag2O and TiO2. Under an excitation of visible light, the photon energy was neither sufficient to excite the electrons transitions from the VB to CB of TiO2, nor from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of adsorbed molecules. However, the presence of surface defects on TiO2 can make the electrons in TiO2 valence band (VB) be excited to surface state energy levels (Ess) by the incident light with the sub-band gap energy and then inject into the LUMO of the adsorbed analytes.22 The transferred electron would eventually transit back to the semiconductor and recombine with the hole in the VB of TiO2. Thus, the dominant contribution to SERS of TiO2 is proposed to be associated with the TiO2-to-molecule charge transfer (CT) mechanism, i.e. photon-induced CT (PICT). Ag2O is a p-type semiconductor with a narrow energy band gap of 1.46 eV. Under visible/infrared-light irradiation, Ag2O can be excited to produce photogenerated h+ and e-. Owing to the p-n junction heterostructure and energy band match of Ag2O and TiO2, the photogenerated electrons excited from the energy level of Ag2O would transfer to the conduction band (CB) of TiO2, while the photogenerated holes remain on the Ag-doping energy level


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(Figure 3g). Therefore, Ag2O can work as an efficient cocatalyst to promote the electron-hole separation and interfacial charge transfer. Photoelectrochemical studies demonstrated that Ag2O even had the greater capacity to transfer electrons to TiO2 than Ag nanoparticle under visible light.23,24 Thus, it is reasonable for us to extend such photocatalytic theory to chemical enhancement mechanisms of SERS/SEIRAS, where the additional large number of photogenerated electrons from Ag2O can take part in the TiO2-to-molecule PICT mechanism. Meanwhile, the heterostructure of Ag2O/ TiO2 can increase the thermodynamically allowed transitions in the PICT process because of both the VB and CB of Ag2O lie above that of TiO2,23 thus enhancing Raman/IR signals. Note that electromagnetic enhancement may be also responsible for the SEIRAS activity, since the surface plasmon resonant frequency of TiO2 is located in the infrared region.25 The SEIRAS EF obtained from our semiconducting substrate were of the same order of magnitude as those achieved on noble metal-based SEIRAS substrate, e.g. Ag and Au nanoparticle film.26 On the other hand, the more Raman-active analyte (4-MBA) had higher SERS EF but lower SEIRAS EF, and more IR-active analyte (BSA) had lower SERS EF but higher SEIRAS EF. This finding suggested that the enhancement of this substrate was greatly dependent on the chemical nature of the probe analytes. These complementary and enhanced spectra implied that our substrate based on Ag2O/TiO2 nanocomposite film can provide complete surface-enhanced spectroscopic analysis of a given sample.


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Raman intensity (a.u.)

4-MBA on TiO2

10000

150

1200

1600

BSA on Ag2O/TiO2 BSA on TiO2

c 50

EF of SERS (105)

b

4-MBA on Ag2O/TiO2


a

g

2.0

Visible light

1.5

Ag2O VB

1200 1400 1600 -1 Raman shift (cm )

1800

d 4-MBA on Ag2O/TiO2 4-MBA on TiO2

1200

e

1400 1600 Raman shift (cm-1)

1800

BSA on Ag2O/TiO2

0.005

4-MBA BSA

f

120

EF of SEIRA

1000

e-

CB

0.2

0.0

80

e- e- e- eCB

e- e-

Ess

eLUMO

Visible light

0.01

BSA on TiO2

IR absorption

IR absorption

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VB

e- e- e-

HOMO

TiO2

Adsorbate

40

0

1000

1200 1400 1600 -1 Wavenumber (cm )

1800

1200

1400 1600 -1 Wavenumber (cm )

1800

4-MBA BSA

Figure 3. Raman (a, b) and IR (c, d) spectra of 4-MBA and BSA adsorbed on pure TiO2 and Ag2O/TiO2 nanocomposite film, and the corresponding EF (c, f). Insert is the magnified view of 4-MBA Raman spectra on pure TiO2. The concentration of analytes (4-MBA and BSA) for SERS/SEIRAS from Ag2O/TiO2 is 10 mg/L, while the concentration for normal Raman and IR from TiO2 is 10 and 0.1 g/L, respectively. (g) Schematic view for energy band matching of Ag2O/TiO2 heterostructure under visible light and charge transfer between the adsorbed molecule and TiO2. The profile of Raman/IR response to 4-MBA/BSA over a wide concentration range were shown in Figure S4. 4-MBA had higher Raman response with a concentration detected as low as 0.01 mg/L (SERS) and 1 mg/L (SEIRAS), respectively. However, this trend was not followed by the more IR-active BSA which can be detected as low as 0.1 mg/L (SEIRAS) compared to 1.0 mg/L (SERS). This difference suggested the dual response of our substrate depending on the chemical nature of adsorbed species. Previous studies revealed that the amount of analytes were linearly correlated with their signals of ν (COO-) mode (4-MBA) and amide I and amide II (BSA), respectively. Therefore, we plotted the Raman intensity and IR second derivative absorbance of the corresponding peaks as a function of analyte concentration (Figure 4 and Figure S5). It was


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noted that with increasing concentration the Raman signals of both BSA and 4-MBA greatly increased and finally reached a steady state. Whereas their IR absorption progressively increased within the tested concentration range. This may be due to the fact that SERS and SEIRAS exhibit different dependences upon the distance to the surface.27 SERS is a short-range effect whereby most of the enhancement of vibrational modes is in the monolayer on the surface, whereas SEIRAS is a longer-range effect that can often lead to vibrational enhancement of a number of adsorbed layers. In our case, such distinct surface distance-dependence can well explain why the Raman intensity reached saturation when the molecules were not close to the surface at high concentration while the IR absorption still increased. The linear behavior of SEIRAS response from our substrate was similar to the observations from Ag and Au nanostructured substrate that surface-enhanced IR absorbance will increase linearly with increasing adsorbed concentration of the analyte7,10. Another reason for the broader quantitative range of SEIRAS could be the higher cross section of the IR absorption compared to the Raman scattering.28 More interestingly, the substrate displayed different quantitative capacities to the two analytes. For 4-MBA, the linear relationship ranged from 0.01-7 mg/L and 7-100 mg/L for SERS and SEIRAS, with a coefficient of determination (R2) equal to 0.978 and 0.983, respectively. For BSA, the linear relationship ranged from 1-10 mg/L and 0.1-30 mg/L for SERS and SEIRAS with R2 of 0.992 and 0.997, respectively. Note that although 4-MBA had lower EF of SEIRAS, its linear response of IR absorbance was observed up to 100 mg/L, much higher than 30 mg/L of BSA. One proposed fact was that the larger molecular structure of BSA probably generated thicker films on the substrate than 4-MBA and therefore, the signal was easier to reach saturation at high concentration. It further implied the dual selectivity of the substrate to enhance and quantify a specific probe analyte.


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-8e-5 -1e-4

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Figure 4. Calibration plot of the SERS/SEIRAS response versus the concentration of 4-MBA and BSA. Inset is the corresponding Raman spectra and 2nd derivative transformation of IR at the peaks that are selected for quantification analysis. Besides sensitivity and quantitative capability, one critical factor for the real applications of the substrate is the good signal reproducibility produced from a uniform surface roughness. To evaluate that, Raman and IR mappings of 4-MBA and BSA have been obtained and analyzed (Figure 5). One hundred spots were selected from a large area with a dimension of 450 µm in length and 450 µm in width. The standard deviation of 4-MBA in SERS and SEIRAS enhancement σ were calculated to be 8.21% and 9.26%, respectively. In the case of BSA, σ were respectively 9.32% in SERS and 7.59% in SEIRAS. The lower standard deviation than 10% indicated that both SERS and SEIRAS spectra were highly reproducible at different sites


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regardless of the adsorbates. Besides, such low spot-to-spot variance well demonstrated that the Ag2O-TiO2 nanocomposites deposited on PVDF membrane can be used as a reliable, uniform and reproducible substrate for both SERS and SEIRAS.

Figure 5. SERS (a, b) and SEIRAS (c, d) mapping (step size 50 µm, mapping area: 450 µm × 450 µm =100 points) of 4-MBA (a, c) and BSA (b, d) with a concentration of 100 mg/L. The brightness is proportional to the 4-MBA Raman signal intensity and IR second derivative absorbance at 1582 cm-1, and BSA Raman signal at 1676 cm-1 and IR second derivative absorbance at 1590 cm-1. (e-h) are the corresponding sectional views of the two crossed lines. The low standard deviation (σ) estimated from the 100 points indicates a uniform and reproducible SERS/SERIAS active sensor. CONCLUSIONS We have demonstrated a powerful 3D substrate based on the Ag2O/TiO2 nanocomposite heterostructure coupled with a portable membrane support, which can provide an innovated, dual functional platform for chemical sensing applications by enhancing both Raman and IR spectroscopy. The remarkable enhancement factors of one or two orders of magnitude can be


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achieved over traditional semiconducting materials and are comparable to noble metal. We proposed that the mechanism for this enhancement is due to the synergetic effect of Ag2O and TiO2 involving the enrichment of electron on the TiO2 surface and the resulting photon-induced charge-transfer enhancement. Importantly, such hybrid semiconducting substrate allowed the detection of ultra-trace levels of various analytes and distinct quantitative capacities for SERS and SEIRAS. High reproducibility of the SERS/SEIRAS spectra confirmed the reliable and reproducible spectroscopic method in our study. Integrating SERS and SEIRAS on a single substrate will enable more detailed investigations of molecular structure, orientation, conformation and adsorbate-substrate interactions. Furthermore, the first discovery that hybrid semiconducting substrates can lead to SERS and SEIRAS simultaneously open new opportunities for the broader development of surface enhanced vibrational spectroscopy techniques to non-metallic surface and potential photoelectrochemical applications. Future study will focus on further improvement of the sensitivity by optimizing the Ag2O concentration and the Ag2O/TiO2 structure, as well as demonstrating more applications of the SERS/SERIAS substrate. REFERENCES (1)

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