Synergistic enhancement effect of MoO₃@Ag hybrid nanostructures for boosting selective detection sensitivity

Highlights

• The self-assemble of MoO₃-NWs@ Ag-NPs as SERS substrate;

• MoO₃-NWs@ Ag-NPs shows a very high detection sensitivity of 10⁻⁸ M (4.79 ug/l) for R6G.

• Rich protuberance of surface and polycrystalline structure lead to EM and CM become synergistic;

• Selective SERS enhancement occurred with different probe molecules.

Abstract

An ex situ method was used to synthesize noble metals and metal oxide composite materials, due to the selective adsorption properties of metal oxides, the adsorption of different probe molecules by this composite structure had been studied. In the ex situ approach, we use (3-aminopropyl) diethoxy methylsilane (ATES) as a coupling agent which is easy for noble metal nanoparticles deposited on metallic oxide nanomaterials. The Raman scattering (SERS) substrate of 1D MoO₃ nanowires (MoO₃-NWs) @Ag nanoparticles (Ag-NPs) hybrid surface had been fabricated. Several parameters are presented in the following which influences the morphology of self-assembly and SERS activity: (i) coupling agent of ATES, (ii) ATES content (iii) Ag-NPs content. The finite difference time domain (FDTD) method is to explain the enhancement mechanism distribution of the hybrid substrate. Different probe molecules (R6G, Methylene Blue, Crystal Violet, and 4-ATP) have been adsorbed for SERS tests. Improved principle component analysis (PCA) is adopted to obtain the minimum detection limit of probe molecules. Through the DFT calculation, different absorption strengths between the target molecules and the MoO₃(010) surface have been illustrated, which is also the main reason for the selective enhancement effect of MoO₃@Ag hybrid nanostructures. This paper might propose a method to prepare such enhancement substrate based on the selective absorption properties of oxide semiconductors.

Introduction

Surface-enhanced Raman scattering (SERS) is a highly sensitive and selective tool for the identification of biological or chemical analytes based on Raman scattering [[1], [2], [3], [4], [5], [6], [7], [8]]. SERS technology has been turned into a powerful and widely used analytical detection method in various fields due to the advantages of highly sensitive detection and molecular fingerprint signals [[9], [10], [11], [12], [13]]. There are two types of SERS effect mechanisms in current theoretical studies: electromagnetic enhancement (EM) and chemical enhancement (CM) [[14], [15], [16], [17], [18], [19]]. On the whole part the former is the electromagnetic resonance excitation of local surface plasmon [20,21], while the latter is brought by the interaction between organic molecules and proximal metal structures.

The analyte in SERS detection must be adsorbed on a SERS active substrate, then analyzed with a Raman spectrometer [[22], [23], [24]]. The behavior of molecules, and the surface configuration of molecules could impact the molecular adsorption behavior of molecules on the substrate surface, than affects the vibration enhancement of SERS substrates [25,26]. Thus, metal oxides with good adsorption level are taken into consideration as SERS substrate.

Metal oxides, due to their properties including wide range of sources, low cost, good stability and selective absorption had been specially attended by SERS researchers [25,[27], [28], [29], [30], [31]]. When the metal oxidized materials are adopted for a SERS substrate, their size, shape, morphology and surface defects could be regulated [[32], [33], [34]]. Because many “hot spots” in the detection could not be produced, the SRES activity would be significantly weakened, thereby the low examination sensitivity would occur. The preparation of a new SERS substrate, composed of heterostructure integration of metal oxides (SnO₂, ZnO, TiO₂ [[35], [36], [37]] etc.) and noble metals, has been done to achieve a stronger Raman signal. As for noble metals (Au, Ag) have also been given much attention.

Compared with other precious metals like Au, Ag has much stronger and more tunable surface plasmon resonance (SPR) in visible and near infrared spectra [38,39]. The surface plasmas on SERS substrates' surface could be produced to prompt SERS active. MoO₃ among transition metal oxides, is an n-type semiconductor with a wide bandgap (E_g) of 3.15 eV [[40], [41], [42]], which has so many unique properties and is widely used as one of the most popular catalysts in the making of electrochromic and photochromic gas sensors [40,[43], [44], [45]]. MoO₃ could be turned into one-dimensional (1D) nanowires through regulated growth conditions to be used as efficient supports in a nanocomposite system. Through the emote SERS technology and the theoretical quantum chemical method found the CM of SERS on 1D MoO₃ belt can reach up to 10³ [46]. The composite structure of noble metals and metal oxide nanowires could help to achieve both CM and EM enhancements [[47], [48], [49]] as well as a specific enhancement due to the selective absorbability of the oxide surface [48,50]. Many methods have been developed to deposit noble metal nanoparticles onto the surface of metal oxides, such as solgel and in-situ growth. Limitations are also existed in these synthetic process, hydrothermal synthesis usually brought a cumbersome process, and thermal evaporation are putting more requirements on experimental conditions [51,52]. The morphology of the composite material by noble metal nanoparticles and metal oxides would be affected, and it will also affect the SERS substrate's activity [53].

In this paper, we have taken example of 1D MoO₃-nanowires (MoO₃-NWs) @Ag-nanoparticles (Ag-NPs) hybrid synthesized in an ex situ method; (3-aminopropyl) Diethoxy methyl silane (ATES) is adopted as a coupling agent. Through regulating the growth conditions MoO₃ could be turned into one-dimensional (1D) nanowires, meanwhile the highly specific attachment area of noble metal nanoparticles would also be made. By adjusting the quality of Ag and ATES, we studied the morphology and SERS activity. The finite difference time domain (FDTD) method was used to simulate the enhancement mechanism distribution of the hybrid substrate. In addition, we have studied the MoO₃@Ag adsorbed on different probe molecules (R6G, MB, CV, 4-ATP), the results of which are theoretically shown by a simulation mode.