**2. Environmental applications of plasmonic-TiO2 nanohybrid**

In this section, plasmonic-TiO2 nanostructure-driven environmental applications have been explained in detail. Environmental applications include the SERS-based detection of pollutant molecules solution and the breakdown of pollutant molecules in water under light illumination. This section also explains the underline mechanism responsible for the outstanding performance of plasmonic-TiO2 nanohybrid for photocatalytic water purification and SERS-based detection process. This section will provide an in-depth understanding for the readers and encourage them to develop several plasmonic nanostructures for environmental applications.

### **2.1 Photodecomposition of organic pollutants in water**

Plasmonic-TiO2 nanohybrids have been proven the outstanding photocatalyst which can effectively decompose industrial pollutants [20–22]. Plasmonic-TiO2 nanohybrids photocatalyst with efficient charge separation properties reveals the light-harvesting ability from UV to visible regions, which make them fascinating for photodecomposition reactions. Various research groups studied the modification of plasmonic nanoparticles over different morphologies of TiO2 for photocatalytic applications [34–36]. The attachment of plasmonic nanoparticles (Ag, Au) with TiO2 nanostructures is an effective way to improve their photodecomposition behavior. The existence of plasmonic nanoparticles on nanostructured TiO2 forms the Schottky junctions interface and captures the photo-generated electrons, and decreases the rate of recombination in TiO2. Moreover, due to the presence of plasmonic nanoparticles

over TiO2, their light-harvesting ability significantly extended [20, 21, 37]. Previous reports indicate the remarkable improvement in TiO2 nanostructures with the modification of plasmonic nanoparticles such as Ag and Au [20–23, 34–37].

Uniformly interspersed Ag nanoparticles on 1-D TiO2 nanostructures have been a favorite choice for their efficient SPR excitation ability, feasibility, electron mobility, and cost-effectiveness. Ag-dispersed TiO2 nanorods [38], nanobelts [39], and nanowires [34] have been synthesized using different synthetic procedures for effective decomposition of a range of dyes such as Rhodamine B, Malachite Green, MB, R6G, etc. A contemporary study conducted by Singh et al. [40] postulates electron scavenging and LSPR of Ag as predominant factors responsible for the exceedingly high efficiency of their synthesized photocatalyst nanocomposites towards toxin degradation. Three different Ag-modified TiO2 nanorods with increasing Ag loading, respectively, were synthesized using one pot wet chemical process. Their optical, structural, and morphological profiles were well characterized. Using sunlight (950 W/cm2 ) as their illumination source, they degraded model organic dyes methyl orange and methylene blue. In another report, Bian et al. [41] informed the synthesis of Au-attached TiO2 nanohybrids using the impregnation reaction and demonstrated photodegradation of chlorophenol, MB, and RhB 4 pollutant solution under Xe lamp exposure. In their photocatalysis studies, 10 μM of chlorophenol, MB, and RhB were decomposed in 4 hours, 20 minutes, and 2 minutes, respectively. Li et al. [42] reported the formation of Au-functioned TiO2 nanostructures by a wet chemical process. Pyrolysis method-driven TiO2 nanostructures were attached with plasmonic Au nanoparticles with various concentrations (2, 5, and 10 wt%). The prepared Au-TiO2 nanohybrids (100 mg) exhibited photodegradation by diminished RhB (20 mg/L) molecules solution in 180 minutes under visible light exposure. Singh et al. [43] reported the Ag-TiO2 thin films formation by combining the sol-gel method with the spin-coating method. **Figure 2(a-d)** show the morphological changes in modified Ag-TiO2 thin films in comparison to the bare Ag-TiO2 and. They tuned the photocatalytic profiles

### **Figure 2.**

*SEM images of (a) bare Ag-TiO2 thin films and (b-d) ion beam modified Ag-TiO2 thin films, (e) UV-Visible spectra of bare and modified Ag-TiO2 thin films, (f) photocatalytic activity of bare and modified Ag-TiO2 thin films under sunlight. Reprinted from the reference [43].*

### *Plasmonic-TiO2 Nanohybrid for Environmental and Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.111524*

of Ag-TiO2 thin films by using the ion-beam irradiation technique. They modulate the optical properties effectively, and consequently, the light-harvesting behavior of the Ag-TiO2 was largely affected. They have observed a significant shift in Ag SPR peak from 456 to 446 nm under different ion fluence values (**Figure 2(e)**). In their photocatalytic activity, they have demonstrated the breakdown of 5 μM MB pollutant molecule solution. They have shown that the most efficient Ag-TiO2 thin film decomposed the MB solution in 30 minutes of sunlight exposure (**Figure 2(f )**).

Wang et al. [44] successfully prepared Au-TiO2 nanohybrids with the joint effort of the sol-gel method with the solvothermal method. In their photocatalytic studies, 500 mg/L of the fabricated photocatalyst was employed for the decomposition of 50 mg/L MB solution under the exposure of a Hg lamp (500 W). In their studies, they have highlighted that Au-modified porous TiO2 nanostructures exhibited superior photodegradation nature as compared to bare porous TiO2. The superior photodecomposition nature of the porous Au-TiO2 nanostructures could be assigned to the effective charge separation and improved light harvesting properties owing to the SPR effect in plasmonic nanoparticles. Recently, Singh et al. [20] reported that Au nanoparticles encapsulated TiO2 nanospheres by using the chemical precipitation method. They have engineered the size of Au nanoparticles by varying the concentration of Au precursors during TiO2 growth. **Figure 3(a-e)** shows the morphology and elemental mapping of Au-encapsulated TiO2 spheres. With the variations in the size of Au nanoparticles, their optical properties improved significantly. Modified Au-TiO2 nanohybrids absorb the entire visible region (500–800 nm) and majorly contributed to increased photocatalytic activity (**Figure 3(f )**). The quenching in the rate of recombination with the variations in the size of Au nanoparticles is further confirmed by the reduction in the intensity of photoluminescence spectroscopy.

They have highlighted that the outstanding photocatalytic behavior of the Au-TiO2 nanohybrids can be assigned to the formation of the Schottky junction interface among the Au nanoparticles and TiO2. Owing to the creation of the Schottky junction, the density of electrons in the conduction band significantly improves the photodecomposition reaction and enhances the degradation rate of the pollutants solution. In their photocatalytic studies, 10 μM of solution three industrial well-known pollutant molecules solution (MB, MG, and MO) were decomposed in 30, 20, and 30 minutes of sunlight exposure, respectively, by using 5 mg of the photocatalyst. Singh et al. [45] prepared the Au-TiO2 thin films by combining the spin coating method with the thermal evaporation method and employed them for the decomposition of a 5 μM solution of MB under natural sunlight exposure. They have demonstrated that 30 minutes of sunlight exposure can decompose a significant amount of pollutants.

Enhanced photocatalytic activity of Au-TiO2 nanohybrids thin film can chiefly correspond to different electronic transfers occurring at the composite surface (**Figure 4**). When Au-TiO2 photocatalyst with adsorbed MB molecules is illuminated by a light source of appropriate wavelength, two major electron pathways get activated. Primarily, upon excitation, electrons from the valence band of TiO2 gain energy and subsequently get transferred to its conduction band, leaving a hole behind and thus generating a pair of photo-induced charge carriers. The presence of Au nanoparticles in the vicinity helps the composite system by taking up the photoinduced electrons from the conduction band of TiO2, thereby suppressing recombination and felicitating catalytic reactions at the surface by acting as an electron sink. Secondly, Au nanoparticles upon light interaction, exhibit surface plasmon resonance (SPR), driving the generation of increased electron-hole pairs in TiO2 on account of intense electric fields localized around these plasmonic nanostructures close to the

### **Figure 3.**

*FESEM images of (a) Au-TiO2 nanohybrids revealing the Au encapsulated TiO2 spheres, (b) combined mapped FESEM image of Au-TiO2 nanohybrids, elemental mapping showing the individual distribution of (c) Au, (d) Ti and (e) O, (f) UV-DRS curves for synthesized Au-TiO2 nanohybrids with tuned optical properties, (g) Scheme for enhanced photodecomposition of Au-TiO2 nanohybrids. Reprinted from the reference [20].*

TiO2 surface. This phenomenon is referred to as Plasmon Resonance Energy Transfer (PRET) [45]. These plasmonic nanoparticles have been reported to serve as "hot spots" allowing increased electron-hole generation and improved photocatalytic reactions [45]. Though Au-TiO2 electron exchange is an important step towards improved catalytic behavior of the nanocomposite system, it is not the key rate-determining step for the reaction. As a consequence of Schottky junction formation, electronic flow is maintained from TiO2 to Au until equilibrium is attained. [45] Apart from Au, the adsorbed pollutant molecules also contribute to increased photocatalytic activity by absorbing light leading to electron transfer into the TiO2 conduction band. The photo-generated electrons produced through different pathways react with oxygen molecules at the surface, converting them into superoxide radicals (•O2 − ), while the holes in the valance band react with water molecules generating hydroxyl radicals (•OH). These unsaturated highly active radicals transform the pollutant molecules into their degradation products [20, 21].

*Plasmonic-TiO2 Nanohybrid for Environmental and Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.111524*

### **Figure 4.**

*Schematic picture revealing the efficient charge transfer among Au and TiO2 for enhanced photodecomposition. Reprinted from the reference [45].*

From the above discussion and excellent studies, it can be concluded that the tremendous properties of plasmonic-TiO2 nanohybrids have significantly improved the photodecomposition process by providing additional light adsorption and reduction in the recombination rate.

### **2.2 Detection of pollutants in water**

The contamination of pollutant molecules in the form of organic dyes, pesticides, pharmaceuticals, and biomolecules requires proper monitoring due to the serious impacts on human health, aquatic life, and the terrestrial environment [46]. As a fingerprint recognition technique, Raman spectroscopy reveals deep chemical and structural insights such as chemical bonding and molecular interactions and thus enables the identification of unknown molecular species [47]. The Raman spectral features consist of bands corresponding to vibrational or rotational transitions; however, its direct applicability is hindered due to inherently weak Raman scattering. Surface-enhanced Raman spectroscopy (SERS) has proven its uniqueness as a reliable spectroscopic technique for the direct identification of bio-molecules and chemicals owing to its selectivity and sensitivity for the sensing of ultralow concentrations of analyte molecules [48]. Nowadays, As a sensitive, specific, non-destructive, and label-free identification technique, SERS has shown enormous potential in several fields inducing medical sciences, life sciences, and analytical chemistry for the targeted identification of chemical and biological hazardous trace species. Primarily, the overall signal enhancement through SERS is governed by electromagnetic as well as chemical enhancement mechanisms. The modified SERS spectral features indicate the adsorption or chemical interaction of analytes with suitable SERS substrates and, thus, imply the involvement of charge transfer-initiated chemical enhancement along with the predominated electromagnetic enhancement. The formation of the charge-transfer complex is critically dependent on the atomicscale substrate's surface properties along with the surface density of the adsorbates; therefore, the chemical state and topography of the surface strongly determine the overall SERS enhancement [49, 50].

### *Updates on Titanium Dioxide*

Initially, a wide range of isotropic and structurally modified anisotropic noble metallic nanostructures such as nanorods, nanotriangles, nanowires, pyramids, nanostars (NSs), nanocubes, nanodendrites (NDs), nanoflowers (NFs), and coreshell NPs in the form of nano-crystalline aggregates and roughened surfaces were utilized for the ultra-sensitive sensing of different analytes. Along with efficient signal enhancement capabilities, the stability, reproducibility, cost-effectiveness, biocompatibility, and spectral background of SERS substrates are the vital parameters restricting the reliable and long-term practical applicability of plasmonic nanostructures-based SERS platforms for the identification of analyte molecules [51]. Owing to their high surface area, excellent physical properties, and flexible and low-cost fabrication peculiarities, semiconductor materials have shown the potential to address the issues with noble plasmonic SERS substrates and recently emerged as promising platforms for SERS-based molecular detection applications utilizing the chemical enhancement and the associated charge-transfer between the analyte and semiconductor's surface. The signal enhancement produced by semiconductor SERS (SC-SERS) substrates relies on the chemical enhancement mechanism and depicts lesser enhancement than traditional plasmonic materials [52]. However, semiconductor SERS substrates recently gained significant research attention as the prospective potential candidates with improved SERS performance. The high sensitivity and resolution of the SERS technique enable the study of the chemical and physical nature of the interaction among molecule and semiconductor nanostructures, as well as the properties of semiconductor nanomaterials. Direct proximity among the adsorbate molecule and SERS substrate initiates the charge transfer (CT) between them and contributes to some distinct spectral features [53, 54]. The varied adsorption direction of the adsorbate molecules on the SERS substrate could produce distinctly resolved spectral features, which in turn provide significant insights into charge-transferinduced chemical enhancement mechanisms.

Interestingly, the synthesis simplicity, high stability, chemical inertness, nontoxicity, and corresponding bio-compatibility of nano-structures titanium (IV) oxide (TiO2) make him a potential candidate for stable SERS signal requirements along with the excellent SERS response. A rich variety of novel TiO2 NPs-based SERS substrates have been fabricated through varying the nanostructure's morphology, including spherical nanoparticles (NPs), nanowires [NWs], and nanorods [NRs] employing the various fabrication strategies such as sol-gel, hydrothermal, and solvothermal methods [55]. Especially, the three-dimensional (3D) architectures such as nanoflowers [NFs] and porous nanomaterials provide the supply larger surface area for the adsorption of analyte molecules and further support the chemical enhancement in SERS. The limited Raman intensity enhancements from semiconductor TiO2 substrates can be improved by optimizing the micro/nano-structured morphology or more efficiently through incorporating plasmonic nanomaterials. Musumeci and coworkers reported the strong SERS signal enhancements of adsorbed enediol molecules over the pure TiO2 NPs surface through the essential formation of the charge-transfer (CT) complex of TiO2 with the molecule. The enhancement was attributed to the increased surface atoms and the consecutive adsorbed analytes molecules as well as borrowing the intensity from the allowed continuum state transitions (**Figure 5**) [56].

Anatase and rutile phases of TiO2 NPs-based SERS substrates were prepared by Yang and coworkers utilizing the sol-hydrothermal method and explored for the TiO2 phase-dependent study towards the sensitive identification of 4-mercaptobenzoic acid probe molecules significantly contributed by the TiO2-to-molecule charge transfer mechanism [57].

*Plasmonic-TiO2 Nanohybrid for Environmental and Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.111524*

### **Figure 5.**

*(a) Absorption, and (b) normal Raman (0.1 M ligands) and SERS spectra of 5 nm pure TiO2 NPs (1 × 10−4 M NP colloidal solution) after the modification with different (4 × 10−2 M) ligands. In figure λexc represents the laser excitation wavelength. Reprinted from the reference [56].*

As shown in **Figure 6**, the charge-transfer initiated SERS activity of analyte molecule adsorbed TiO2 NPs is dominated by the three different kinds of transition (i) the excitation of electrons from the valence band (VB) to the conduction band (CB) of TiO2 and their subsequent transition to LUMO of the adsorbed molecules with excitation larger than the TiO2 band gap energy (~3.2ev), (ii) the excitation of electrons from HOMO to LUMO of adsorbed molecules followed by the injection into TiO2 CB, and (iii) the excitation of electrons from TiO2 VB to surface state energy levels and their transfer to the LUMO of the adsorbed molecules [58]. The abundance availability of TiO2 surface defects or surface oxygen vacancy defects can bind the electrons and result in the formation of surface state energy levels. Lin et al. reported

### **Figure 6.**

*Different modes representing the charge transfer between the TiO2 and adsorbed analyte molecule. Reprinted from the reference [58].*

the ultrahigh SERS activity with EF ~105 for the 4-NBT probe molecule utilizing photo-induced charge transfer employing crystal-amorphous core-shell black TiO2 NPs by a controllable solid-state synthesis method [59]. Electrochemical anodization of titanium substrates was employed for the fabrication of TiO2 Nanotube Arrays for the improved detection of cytochrome C with EFs ~104 and LOD down to the ~10−7 M concentration [59]. Notably, the crystalline femtosecond laser generated TiO2 nanofibers in the form of three-dimensional chained nanoparticles provided the resultant EFs of ~1.3 × 106 of the CV dye molecules, which is comparable enhancement produced by noble metal-based substrates. This significant contribution to SERS is assigned to the fabrication of 3D nanonetworks, including the combined contribution of nanogap, nanocluster, and plasmonic hybridization [60].

Nowadays, the noble metal-semiconductor nanocomposites have attracted significant research attention as noble SERS substrates utilizing the synergistic interaction between the noble metal and semiconductor surfaces. The relatively poor SERS activity of pure TiO2 can be significantly improved by incorporating plasmonic materials with TiO2 substrates through the composite formation. Notably, noble metal NPs can be simply incorporated on the TiO2 surface through the photocatalytic process and therefore offers the ease of fabrication of noble metal-TiO2 hybrid nanocomposites [61]. The SERS activity of semiconductor substrates is dominantly assigned to metalmolecule charge-transfer contribution and the correspondingly involved chemical enhancement mechanism results in the weak SERS activity of pure TiO2 NPs-based substrates. The considerably increased photophysical characteristics can be achieved through the attachment of metal nanoparticles with TiO2, through moving the titania's Fermi level to more negative potentials and inhibiting the electron-hole pair recombination [62].

Initially, some studies reported the fabrication and SERS performance of hybrid Ag/TiO2 and Au/TiO2 composites [55]. The excitation of SPR of AgNPs produces enhanced electromagnetic fields at the AgNPs surface and, in turn, the generation of hot electrons at the metal-semiconductor interface. The correspondingly formed Schottky junction promotes the transfer of generated hot-electrons towards the conduction band of TiO2 from the metal-semiconductor interface. Thus, the SERS activity of Ag/TiO2 hybrid substrates is mainly governed by the three contributions, namely, the plasmonic NPs, the transition of electrons between the highest occupied molecular orbital (HOMO) energy level, and the lowest unoccupied molecular orbital (LUMO) energy level of analyte molecule and initiation of charge transfer between the Fermi level of Ag nanoparticle and HOMO level of the analyte molecule, as explained by Zhao et al. for adsorbed R6G molecules over surface of Ag/TiO2 nanowire substrates [63].

Fu at el. employed the modified photocatalytic method for the AgNPs deposited TiO2 films for the detection of rhodamine 6G dye molecules [64]. However, the fast photocatalytic process results in the inhomogeneous synthesis of AgNPs under UV exposure and so restricts the applicability of the photocatalytic method for reproducibly uniform deposition of AgNPs over TiO2 surface. Especially, Song and coworkers demonstrated the applicability of metal composite nanofibers by detecting 4-MPy molecules adsorbed over the AgNPs-coated TiO2 nanofibers by an enhancement factor of ~105 [65]. In another report, Wang and coworkers fabricated the in-situ AgNPsdeposited well-ordered TiO2 nanosheets utilizing hydrothermal synthesis exhibiting excellently uniform and renewable SERs activity with EF ~108 for 4-mercaptobenzoic acid [66]. Yang et al. utilized the wet chemical approach for the preparation of Ag/ TiO2 core-shell NPs and reported higher sensitivity as compared to Ag cores through

### *Plasmonic-TiO2 Nanohybrid for Environmental and Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.111524*

synergistic SERS enhancement. The prepared ultra-thin coated Ag/TiO2 core-shell NPs were employed for the 4-MBA and tetramethyl thiuram disulfide (TMTD) insecticide with significantly lowered detection limits of 10−11 M and 10−10 M concentrations, respectively [67]. Notably, Lu et al. synthesized the hierarchical micro/ nanostructured TiO2/Ag architectures utilizing a combined approach of femtosecond laser structuring and hydrothermal treatment. The prepared SERS substrates demonstrated four-fold enhancement compared to the hydrothermally fabricated bare Ti surface along with LODs down to ~10−14 M R6G concentration [68]. In addition, the applicability of Ag/TiO2 hybrid SERS substrates was also explored towards the detection of various biomolecules such as antibiotics, circulating tumor cells (CTC), uracil DNA glycolase (UDG), and bacteria E. coli and S. aureus [69–72]. Recently, photo-catalytically synthesized TiO2@Ag nanostructured SERS bioprobe substrates were developed by Xu et al. targeted towards the ultralow (~10−14 M) concentration detection of R6G molecules and further utilized to specifically recognize CTC with LOD down to 1 cell per mL (**Figure 7**) [73].

Therefore, the recent SERS studies unveiled the amplified SERS performance of hybrid Ag/TiO2 substrates comparable to noble plasmonic NPs based substrates with detectability down to picomolar and femtomolar concentration range.

Furthermore, TiO2 substrates coated with Au nanohybrid structures were widely utilized to obtain higher sensitivity, stability, and superior compatibility for detecting biological molecules. The incorporation of Au with TiO2 substrates improves the biocompatibility of the substrates along with the promoted stability of the SERS signal due to the non-oxidizing properties of Au. For instance, Jiang et al. proposed Au-deposited TiO2 (Au-TiO2) nanocomposites through photocatalytic reduction of HAuCl4 on TiO2 NPs [74]. The prepared Au-TiO2 nanocomposites produced recyclable and sensitive SERS detection of 4-MBA molecules with the lowest detectable concentration of ~10−8 M, which benefitted from the synergistic contribution of metals and semiconductors. The employment of nanometer-sized inter-particle gaps with densely packed AuNPs deposited on vertically aligned large-area TiO2 nanosheets (NSs) reported superior sensitivity along with the ultralow detectability of ~10−14 M crystal violet concentration (**Figure 8**) [75].

Interestingly, Singh et al. employed the combined thermal deposition and spin coating procedure with the improved optical response and targeted towards efficiently sensitive and simultaneous identification of two organic pollutants rhodamine

### **Figure 7.**

*(a) SERS signal of different concentration of R6G molecules adsorbed over TiO2@Ag nanostructure. The inset depicts the SERS spectra at low R6G concentrations (10−12, 10−13, and 10−14 M), and (b) SERS intensity at ~613 cm−1 R6G peak adsorbed over TiO2@Ag–R6G SERS system at different R6G concentrations. Reprinted from the reference [73].*

**Figure 8.**

*(a) SEM image depicts the TiO2 NS/Au NPs@SiO2/Au NPs sample, and the inset shows a typical SEM image, and (b) SERS spectra of CV molecules with TiO2 NS/Au NPs@SiO2/Au NPs-based substrates. Reprinted from the reference [75].*

6G (R6G) and methylene blue (MB), exhibiting the Raman intensity enhancement factors of the order of ~107 along with excellent multiplexed detection capabilities [45]. Photo-reduced growth of AuNPs over TiO2 (TiO2/AuNWAs) nanowire arrays used as recyclable and sensitive SERS platform for detecting ~10−9 M concentration of R6G molecules. In addition, the comparative SERS study of bare TiO2, AuNPs, and TiO2/Au NWAs reveal remarkably improved SERS performance of TiO2/Au NWAs substrates [76]. Recently, AuNPs decorated ordered porous TiO2 thin films based SERS sensors were optimized by Liang and coworkers for detection of rhodamine 6G probe molecules [77]. The improved bio-compatibility of Au/TiO2 nanostructures offers an additional degree of freedom for detecting various biomolecules with high sensitivity. The highly sensitive detection of adenine biomolecules was performed by Jiang and coworkers utilizing Au/TiO2/Au nanosheet substrates through sputtered deposition of AuNPs with a detection limit of ~10−7 M concentration [78].

Apart from monometallic NPs deposited TiO2 SERS substrates, bimetallic Au/Ag nanoparticles (NPs) decorated well-aligned TiO2 nanorod arrays (NRAs) with strong absorption from 400 to 1300 nm spectral region generated the superior NIR-SERS activity for antibiotic chloramphenicol and ciprofloxacin in real-world water samples with nano-molar detection limits [79]. Additionally, Borges et al. demonstrated the incorporation of bimetallic Ag-Au NPs with TiO2 and the strong LSPR band and correspondingly improved SERS activity [80]. Along with noble (Ag and Au) metals, some recent SERS studies also focused on the implementation of poor plasmonic materials for the development of hybrid TiO2 substrates. To prove the applicability of non-plasmonic materials, the uniformly deposited PtNPs onto the vertically aligned TiO2 nanorod arrays (Pt@ TiO2 NTAs) for the detection of R6G through effective prevention of electron-hole pairs recombination [81]. In another study by Jiang et al., the SERS activity of Fe2O3@TiO2 hybrid substrates was investigated employing exosomal miRNAs via integrated miRNAtriggered hot-spot SERS and Fe3O4@TiO2-based exosome accumulation [82].

In summary, the excellent physical, chemical, and biological properties of metaloxide TiO2 semiconductor enables broad SERS detection applications through facile incorporation of plasmonic materials utilizing synergistic interaction between TiO2 and metallic NPs. The SERS substrate design, material composition, and synthesis strategy strongly govern the stability, reproducibility, and compatibility with biomolecules and subsequently offers a new perspective of SERS substrate development benefitting from the charge-transfer induced chemical enhancement SERS mechanism.
