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. 2021 Oct 11;6(41):27271–27278. doi: 10.1021/acsomega.1c04082

Recyclable Ag-Deposited TiO2 SERS Substrate for Ultrasensitive Malachite Green Detection

Weiye Yang †,‡,§, Junqi Tang †,‡,§, Quanhong Ou †,‡,§, Xueqian Yan †,‡,§, Lei Liu †,‡,§, Yingkai Liu †,‡,§,*
PMCID: PMC8529650  PMID: 34693147

Abstract

graphic file with name ao1c04082_0010.jpg

An ultrasensitive Ag-deposited TiO2 flower-like nanomaterial (FLNM) surface-enhanced Raman scattering (SERS)-active substrate is synthesized via a hydrothermal method, and Ag nanoparticles (NPs) are deposited through electron beam evaporation. Malachite green (MG), which is widely used in aquaculture, is employed to assess the surface-enhanced Raman scattering (SERS) properties of TiO2/Ag FLNMs. They exhibit ultrasensitivity (limit of detection (LOD) of MG reaches 4.47 × 10–16 M) and high reproducibility (relative standard deviations (RSDs) are less than 13%); more importantly, the TiO2/Ag FLNMs are recyclable, as enabled by their self-cleaning function due to TiO2 photocatalytic degradation. Their recyclability is achieved after three cycles and their potential application is examined in the actual system. Finite difference time domain (FDTD) simulations and the charge-transfer (CT) mechanism further prove that the excellent SERS properties originate from localized surface plasmon resonance (LSPR) of Ag NPs and the coupling field between Ag and TiO2 FLNMs. Therefore, TiO2/Ag FLNMs show promising application in aquaculture.

1. Introduction

Surface-enhanced Raman scattering (SERS) was accidentally discovered by Fleischmann and co-workers in 1974,1 which has been rapidly growing over the past 40 years and has now become a valuable technique in the fields of physics, chemistry, medicine, etc.2−4 The main drawback of the application of SERS technology is the use of effective substrates, which not only supply high electromagnetic enhancement but also provide stable, uniform, and reproducible performance.5,6 Several methods have been successfully utilized to improve the performance of SERS substrates. Many of them are made from noble metals with multiple shapes, such as nanorods, nanoparticles (NPs), nanotriangles, nanocubes, and core–shell nanoparticles.7,8 Recently, semiconductor materials have been applied to synthesize semiconductor–noble metal SERS substrates that have attracted tremendous attention due to their low cost, high sensitivity, uniformity, and reproducibility.9−13

TiO2 is a significant wide-band-gap semiconductor that has gained wide attention due to its excellent chemical and optical properties.14 Recently, TiO2 has attracted extensive attention as a suitable candidate as an SERS substrate. It is well-known that Ag NPs provide excellent SERS activity in the visible-light wavelength region and have much lower cost compared with Au NPs, which is very suitable for SERS substrates.15 Therefore, it is a reasonable attempt to deposit Ag NPs on the surface of TiO2. Combining TiO2 and Ag NPs as a SERS substrate would bring multifunctional SERS activity, which can be applied to detect organic pollutants.

Environmental issues attract increasing attention due to organic pollutants and contaminated food.5 Malachite green (MG) is a common organic pollutant in aquaculture as an antiseptic, fungicide, and ectoparasiticide due to its efficiency against parasitic and fungal infections in aquatic products. But it is probably teratogenic and mutagenic and even carcinogenic to humans. Therefore, it is banned in aquaculture. However, it is still abused in aquatic products due to its high efficiency and low cost. Therefore, it is necessary to conduct an MG test in aquatic products.6 The SERS technique is gradually becoming an available tool for detecting environmental pollutants. For its practical application, it is very important to develop SERS-active substrates with low cost and high efficiency. Improvements can be made by the following ways: developing a cheaper technique of fabricating substrates,16 fabricating a multifunctional SERS substrate,17 and designing a renewable SERS substrate.18,19

In this paper, a recyclable SERS substrate was made of flower-like TiO2 with deposited Ag NPs. To assess the SERS performance of TiO2/Ag substrates, MG was chosen as a probe molecule. These newly developed SERS substrates showed great advantages in applications: high sensitivity, strong reproducibility, excellent recyclability, and stability in actual application.

2. Results and Discussion

2.1. Structure and Morphology

Figure S1a,b displays the scanning electron microscopy (SEM) images of TiO2 flower-like nanomaterials (FLNMs), revealing that TiO2 nanomaterials comprise abundant flower-like aggregates and have almost similar morphology. These TiO2 flower-like nanomaterials have sizes in the range of 0.8–1.6 μm (Figure S1e). Figure S1c,d displays the SEM images of TiO2/Ag FLNMs, which are covered with Ag NPs with sizes ranging from 20 to 50 nm (Figure S1f).

Figure 1 shows the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of TiO2/Ag FLNMs. The microstructure of flower-like TiO2/Ag FLNMs is further observed. Figure 1b shows the HRTEM image of Ag NPs with a fringe spacing of 0.25 nm, which corresponds to the (100) plane of hexagonal Ag. Figure 1c shows the HRTEM image of TiO2 with lattice fringes of 0.35 nm, which corresponds to the (101) plane of anatase TiO2. The SEM and TEM images demonstrate that Ag NPs are successfully deposited on the TiO2 FLNMs.

Figure 1.

Figure 1

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(a) TEM images of TiO2 flower-like nanomaterials, (b) the HRTEM image of Ag NPs, and (c) the HRTEM image of TiO2.

Figure S2 portrays the X-ray diffraction (XRD) patterns of TiO2 FLNMs and TiO2/Ag FLNMs. TiO2 FLNMs exhibit the pure anatase phase due to characteristic diffraction peaks at 2θ = 25.28° (101), 37.80° (004), 48.05° (200), 53.89° (105), 55.06° (211), 62.88° (204), 68.76° (116), 70.31° (220), and 75.03° (215). The results can be introduced to anatase TiO2 (ICDD (International Center of Diffraction Data) No. 21-1272). The red line belongs to TiO2/Ag FLNMs. It shows no Ag signals, as the thickness of deposited Ag particles is only 6 nm.

2.2. X-ray Photoelectron Spectroscopy (XPS) Spectrum

To check the chemical valence states and chemical composition of TiO2/Ag FLNMs, XPS measurements were carried out (Figure S3). C 1s at 284.8 eV is selected as the calibration peak. Figure S3a shows the full-scan XPS spectrum. It is observed that TiO2/Ag FLNMs are composed of Ti, O, C, F, and Ag elements. F element is the residual of hydrofluoric acid (HF) from the hydrothermal reaction, and the peak at 684.7 eV indicates that F– is absorbed on the surface of TiO2.20Figure S3b illustrates the XPS band of Ag 3d. Peaks at 367.8 and 373.8 eV are attributed to Ag 3d5/2 and Ag 3d3/2, respectively, indicating that Ag is deposited on the TiO2 nanomaterials. Figure S3c shows XPS bands for the Ti 2p region; peaks at 458.9 and 464.7 eV are attributed to Ti 2p3/2 and Ti 2p1/2, respectively, indicating the presence of Ti4+ ions.21Figure S3d shows the XPS band of O 1s. The characteristic peak of O 1s is composed of two peaks fitted by the Gaussian equation (Avantage, Thermo Fisher Scientific). The peak at 530.8 eV originates from the O–Ti bonding, and the peak at 532.8 eV is attributed to O2 adsorption. The XPS results verify that TiO2/Ag FLNMs are successfully obtained.

2.3. SERS Applications

Illegally used aquaculture drugs cause severe problems in aquatic food products due to their negative effects on the environment and public health. MG is a triphenyl methane dye that has been abused used over the past few decades in aquaculture, with high fungicidal efficiency even at very low concentrations.22 However, MG can enter the food cycle easily and cause carcinogenic, mutagenic, and teratogenic abnormalities in humans.23 Thus, a fast, facile, low-cost, and sensitive analytical technique is urgently needed for detecting MG in aquatic products. The SERS technique provides a new way to fulfill this demand. The SERS properties of TiO2/Ag FLNMs, including sensitivity, reproducibility, and recyclability, and actual application are demonstrated through a series of experiments.

2.3.1. Sensitivity

The SERS performance of TiO2 FLNMs, Ag NPs, and TiO2/Ag FLNMs is demonstrated in Figure 2a–c, respectively. It is obvious that TiO2/Ag FLNMs exhibit the best SERS performance. The fingerprint Raman peaks of MG are still visible even at 10–15 M (Figure 2c). Peaks at 917, 1179, and 1215 cm–1 originate from the out-of-plane C–H bending, the in-plane modes of C–H bending, and C–H rocking, respectively; peaks at 1294 and 1368 cm–1 belong to in-plane aromatic C–H bending vibration and N-phenyl stretching, respectively; and peaks at 1491, 1594, and 1619 cm–1 originate from ring C–C stretching.24−26 All peaks are consistent with the characteristic peaks of MG.

Figure 2.

Figure 2

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SERS signal of various concentrations of MG on (a) TiO2 FLNMs, (b) Ag NPs, and (c) TiO2/Ag FLNMs. (d) The linear fit of the peak at 1619 cm–1 versus MG concentrations for TiO2/Ag FLNMs.

It is noted that the minimum concentration of MG on TiO2/Ag FLNMs reached 1.0 × 10–15 M and the limit of detection (LOD)27 is another prominent parameter for SERS performance that is meaningful for practical application. The intensity of the peak at 1619 cm–1 versus the logarithmic concentration of MG for TiO2/Ag FLNM substrates is illustrated in Figure 2d. The corresponding LOD is 4.47 × 10–16 M.

The enhancement factor (EF) is another significant parameter for SERS. To assess the EF of TiO2/Ag FLNMs, the peak at 1619 cm–1 for MG is employed to evaluate the EF according to the following equation28

2.3.1. 1

where ISERS and IRS represent the Raman intensities of the probe molecules adsorbed on SERS substrates and bare substrates, respectively. CSERS and CRS are the concentrations of probe molecules on SERS substrates and bare substrates, respectively. All of the testing conditions are consistent. A high SERS EF of 3.49 × 1011 is obtained when using a CSERS of 10–15 M and a CRS of 10–4 M. Compared with other substrates, different SERS substrates used for MG detection with their enhancement factor (EF) and LOD (M) are listed in Table S1. This reveals that TiO2/Ag FLNMs achieve ultrasensitive detection.

2.3.2. Reproducibility

The reproducibility of SERS signals is determined under consistent testing conditions; the signals from multiple tests are within a certain error range that is no more than 20%.29 Three-dimensional (3D) Raman signals of MG at 1.0 × 10–13 M are acquired from 30 random spots (Figure 3a) and 3D mapping (Figure S4), which show that excellent reproducibility is achieved. The relative standard deviations (RSDs) of the characteristic peaks at 1179, 1368, and 1619 cm–1 are 9.61, 12.34, and 4.26%, respectively (Figure 3b–d). All RSDs are less than 13%, indicating that TiO2/Ag FLNM substrates have high reproducibility. The reasons for the high reproducibility of TiO2/Ag can be summarized as follows: first, TiO2/Ag FLNMs are uniform and Ag NPs are equally spread on the surface of FLNMs. Furthermore, MG molecules are completely adsorbed before testing, making the concentration of MG consistent on the surface of TiO2/Ag FLNMs. Therefore, TiO2/Ag FLNMs would be a promising substrate for practical applications.

Figure 3.

Figure 3

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(a) Three-dimensional (3D) Raman spectra of 10–13 M MG at 30 randomly spots on the TiO2/Ag FLNMs. (b–d) RSDs values of the selected peaks at 1179, 1368, and 1619 cm–1, respectively.

2.3.3. Recyclability

Due to the photocatalytic degradation of TiO2, the clear advantage of TiO2/Ag FLNMs is its recyclability.5,30 For recyclable SERS performance, Figure 4a displays the typical process: after SERS measurements, the substrates are soaked in deionized water and irradiated with a UV lamp (<400 nm) for 1 h at room temperature, and then the substrates are placed 2 cm away from the UV lamp with the nanostructures facing the UV light. To check if the photocatalytic degradation is completed, SERS detection is used to check that the substrates are clean. In recyclable processes, the photocatalytic reaction of MG is31

2.3.3. 2

The advantage of photocatalytic degradation is that it decomposes MG molecules to CO2, H2O, HNO3, and small molecules, which endows the SERS substrates with the self-cleaning function. The process is repeated three times to ensure its recyclability (Figure 4b), indicating that TiO2/Ag FLNMs are suitable for use as a recyclable SERS substrate.

Figure 4.

Figure 4

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(a) Flow chart of the recyclable SERS behavior of the TiO2/Ag FLNMs. (b) Recyclable SERS behavior of the TiO2/Ag FLNMs with three cycles.

2.3.4. Application in the Actual System

To further verify the actual system of TiO2/Ag FLNMs, various water samples from the Fuxian Lake and the Dian Lake are selected as actual water samples without any purification. Before SERS measurements, 0.0927 g of MG is dissolved in 10 mL of Fuxian Lake and Dian Lake water samples, separately. Then, they are diluted to different concentrations. Figure 5a displays the SERS spectra of MG on TiO2/Ag FLNM substrates from different water samples at a concentration of 1.0 × 10–10 M. It is found that all of the water samples show high SERS performance, and the minimal detectable concentration of MG with the Fuxian Lake water sample is 1.0 × 10–12 M (Figure 5b) and the minimal detectable concentration of MG with the Dian Lake is 1.0 × 10–11 M (Figure 5c). They are lower than the minimum value found in aquatic products (2 μg/kg).32 This proves that TiO2/Ag FLNMs possess potential applicability in actual water systems.

Figure 5.

Figure 5

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(a) SERS spectra of MG on the TiO2/Ag FLNMs from different water samples, (b) MG spectra with the Fuxian Lake water samples, and (c) MG spectra with the Dian Lake water samples.

2.4. Mechanism

It is commonly recognized that there are two interacting mechanisms for the SERS effect: the electromagnetic (EM) mechanism and the charge-transfer (CT) mechanism.33,34 The EM mechanism involves the enhancement of optical fields and requires the excitation of the localized surface plasmon resonance (LSPR) originating from the substrate.35 The EM model does not require specific bonds between the adsorbate and the substrate, which is a long-range effect and is mainly contributed by noble metals.36 The CT mechanism is mainly contributed by by semiconductor substrates, which involves the CT between the adsorbate and the substrate and is a short-range effect.37 A direct bond between the adsorbate and the substrate is required for the CT enhancement. The SERS enhancement mechanism of TiO2/Ag FLNMs is the combined contribution of the EM effect of surface-deposited Ag NPs and the CT effect between TiO2 and probe molecules.

To verify the EM mechanism contribution of TiO2/Ag FLNMs, the finite difference time domain (FDTD) method was applied to simulate the EM field distribution of Ag NPs and Ag NPs deposited on TiO2 FLNMs. In this simulation, the diameter of Ag NPs was set to 20 nm, the separation distance of the adjacent Ag NPs was 5 nm, the thickness of TiO2 layer was 20 nm, and the laser wavelength was 532 nm. The FDTD simulation results are shown in Figure 6, which reveals that the strong EM field is distributed in the space between Ag NPs (Figure 6a) and the interface between Ag NPs and the TiO2 layer (Figure 6b). This tiny gap was called a hot spot that played a decisive role in SERS enhancement. With more hot spots, it was easier to obtain high-intensity SERS signals.38,39 Compared with Figure 6a, the EM field was more stronger, as shown in Figure 6b, which would achieve high SERS performance. From the FDTD results, TiO2/Ag FLNM substrates supplied more hot spots and high EM, which led to improve the SERS performance.

Figure 6.

Figure 6

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FDTD simulations of EM distribution on the TiO2/Ag FLNMs. (a) EM distribution of Ag NPs. (b) EM coupling between Ag NPs and TiO2 FLNMs.

According to the previous work,40 the SERS enhancement mechanism of a semiconductor substrate depends on the chemical adsorption between the SERS substrate and probe molecules. The probe molecules can be adsorbed to the substrate, and its Raman signals can be enhanced by the CT mechanism, even at ultralow concentrations. The molecules can also be effectively identified due to chemical adsorption and CT. Therefore, the semiconductor substrate shows ultrasensitive detection properties at ultralow concentrations.41

It is obvious that the enhancement on TiO2/Ag substrates must be from the synergistic effect of Ag and TiO2 including the EM enhancement of Ag and the CT enhancement of TiO2. The combined SERS enhancement on the TiO2/Ag substrates can be described as follows

2.4.

Here, IEM(Ag) and IEM(TiO2–Ag) represent the EM enhancements of deposited Ag NPs and the interface between Ag NPs and TiO2 FLNMs, respectively. The other three terms in the expression originate from the CT enhancement between Ag NPs, TiO2, and MG molecules. The deposited Ag NPs on the surface of TiO2 can be excited by localized surface plasmon resonance (LSPR), and photoexcited electrons can be injected into the conduction band (CB) of TiO2 after the transfer to the lowest unoccupied molecular orbital (LUMO) of the MG molecules adsorbed on TiO242 (as shown in Figure 7). All these items contribute to the combined TiO2/Ag substrate enhancement together. Therefore, it is not surprising to observe that TiO2/Ag FLNMs exhibit high SERS enhancement in our present experiment.

Figure 7.

Figure 7

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Sketch map of the CT enhancement mechanism of MG on TiO2/Ag FLNMs.

3. Conclusions

In summary, a novel strategy for a multifunctional SERS substrate based on the Ag-deposited TiO2 FLNM substrate is proposed with excellent sensitivity, reproducibility, and recyclability. The developed TiO2 FLNM substrate has been used for the detection of MG, which is a universal organic pollutant in aquaculture. Raman measurements indicate that TiO2/Ag FLNMs have high sensitivity (the LOD of MG reaches 4.47 × 10–16 M), reproducibility (RSDs are less than 13%), and recyclability (it can be repeated for three cycles) and exhibit a promising application in an actual system. Furthermore, the FDTD simulation results reveal that TiO2/Ag FLNM substrates give rise to more hot spots, and the CT process of metal–semiconductor–molecular system synergetically contributes to its high SERS performance. Thus, we firmly believe that TiO2/Ag FLNM substrates have potential to be used in aquatic products.

4. Experimental Section

4.1. Reagents and Instruments

A Ti film (99.999% purity) was supplied by Haiyuan Aluminum Corporation, hydrofluoric acid (HF, 40 wt %) was purchased from Tianjin Fengchuan Chemical Reagent Corporation, and malachite green (MG) was purchased from Maklin Corporation. Deionized water of 18.2 MΩ was used in all experiments. All reagents were used as received without any refinement.

Surface morphology structural properties were characterized by scanning electron microscopy (SEM, Quanta FEG 250, FEI). The microstructures of the substrates were further studied by transmission electron microscopy (TEM, JEOL 2010, Japan). X-ray diffraction (XRD) data were measured using an Ultima IV Rigaku (Japan) to verify the phase of the substrates. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific) was performed to check the chemical valence states and chemical composition. SERS spectra were recorded using a confocal Raman spectrometer (Andor, England).

4.2. Preparation of TiO2 FLNMs (Flower-like TiO2 Nanomaterials)

TiO2 flower-like nanomaterials were synthesized by a one-step sol hydrothermal method between the Ti film and the HF solution according to the precious work.43 First (Figure 8a), a mixed solution of 10 mmol of hydrofluoric acid (HF) and 60 mL of 3 cm × 1 cm Ti films was put into a 100 mL stainless steel Teflon autoclave at 110 °C for 6 h. The chemical processes were as follows.

4.2. 3
4.2. 4
4.2. 5

After natural cooling, the product was rinsed with deionized water and dried at 80 °C for 6 h. Then, TiO2 FLNMs were obtained.

Figure 8.

Figure 8

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Schematic diagram of the preparation of TiO2/Ag FLNMs. (a) TiO2 FLNMs were prepared via the hydrothermal reaction, (b) Ag NPs were deposited via electron beam evaporation, and (c) TiO2/Ag FLNMs.

4.3. Preparation of Hybrid TiO2/Ag FLNMs

After the abovementioned processing, Ag NPs were deposited on films by electron beam evaporation (Chinese Academy of Sciences Instrument Corporation, China). The evaporation rate was 0.1 Å/s and the thickness was 6 nm, Finally, TiO2/Ag FLNMs were obtained (Figure 8b,c).

4.4. SERS Measurements

In the traditional method, probe molecules were first dropped onto the substrate and dried, which greatly increased the detection time. Moreover, during the drying process, the coffee ring distributed the probe molecules nonuniformly, which led to lower reproducibility.44 To overcome this shortcoming, promoting detection efficiency and signal stability, SERS measurements were conducted in the aqueous environment in this paper. First, TiO2/Ag FLNM substrates were soaked in solution for 3 h to achieve adsorption equilibrium. Then, the substrates were transferred to a custom-made quartz slot (15 mm × 15 mm × 1 mm), which was covered with the cover glass (Figure S5). The Raman signals were recorded using a confocal microscopy Raman spectrometer system with an excitation laser wavelength of 532 nm. A microscope objective of 50× was used, 1200 (L/mm) grating was selected, acquisition time was adjusted to 10 s, and the accumulation number was three times for each test.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (Grant Nos. 11764046 and 11764047).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04082.

  • SEM images of TiO2 FLNMs and TiO2/Ag FLNMs (Figure S1); XRD pattern of TiO2 FLNMs and TiO2/Ag FLNMs (Figure S2); XPS analysis of TiO2/Ag FLNMs (Figure S3); schematic of the custom-made quartz slot (Figure S4); and different SERS substrates used for MG detection with their enhancement factor (EF) and LOD (M) (Table S1) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04082_si_001.pdf (977.8KB, pdf)

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