A molecularly imprinted nanoprobe incorporating Cu2O@Ag nanoparticles with different morphologies for selective SERS based detection of chlorophenols

Development and Biomedical Application of Non-Noble Metal Nanomaterials in SERS

Surface-enhanced Raman scattering (SERS) is vital in many fields because of its high sensitivity, fast response, and fingerprint effect. The surface-enhanced Raman mechanisms are generally electromagnetic enhancement (EM), which is mainly based on noble metals (Au, Ag, etc.), and chemical enhancement (CM). With more and more studies on CM mechanism in recent years, non-noble metal nanomaterial SERS substrates gradually became widely researched and applied due to their superior economy, stability, selectivity, and biocompatibility compared to noble metal. In addition, non-noble metal substrates also provide an ideal new platform for SERS technology to probe the mechanism of biomolecules. In this paper, we review the applications of non-noble metal nanomaterials in SERS detection for biomedical engineering in recent years. Firstly, we introduce the development of some more common non-noble metal SERS substrates and discuss their properties and enhancement mechanisms. Subsequently, we focus on the progress of the application of SERS detection of non-noble metal nanomaterials, such as analysis of biomarkers and the detection of some contaminants. Finally, we look forward to the future research process of non-noble metal substrate nanomaterials for biomedicine, which may draw more attention to the biosensor applications of non-noble metal nanomaterial-based SERS substrates.

Porous rod-shaped Fe2O3/Ag/BP: a novel substrate for highly sensitive SERS detection of persistent organic pollutants

A surface enhanced Raman scattering (SERS) substrate of porous rod-shaped ferric oxide (Fe2O3) combined with silver nanoparticles (Ag NPs) and black phosphorus (Fe2O3/Ag/BP) was fabricated to detect the persistent organic pollutants (POPs) at low concentration. The organic pollutant Rhodamine 6G (R6G) was used as the probe molecule to study the performances of Fe2O3/Ag/BP, and 4-chlorobiphenyl (PCB-3) was the target of detection. The limit of detection (LOD) of R6G based on this novel SERS substrate Fe2O3/Ag/BP was as low as 1.0 × 10-15M, which was five orders of magnitude lower than that of Fe2O3/Ag (10-10M). The enhancement factor (EF) of Fe2O3/Ag/BP was 6.44 × 108, which was 3.1 times higher than that of porous rod-shaped Fe2O3/Ag (2.08 × 108). The Raman signal of R6G based on Fe2O3/Ag/BP had a good homogeneity, and the relative standard deviation (RSD) of Raman signal intensities of R6G at 1643 cm-1was only 5.97%. Furthermore, the Fe2O3/Ag/BP substrate exhibited a recyclability through the photocatalytic degradation of R6G. The LOD of PCB-3 based on Fe2O3/Ag/BP was 10-9M. Besides, Fe2O3/Ag/BP had a high SERS activity even it was kept in a centrifuge tube without requiring complicated treatment. These results highlight the potential application of Fe2O3/Ag/BP for ultra-trace detection of POPs in the environment.

Synthesis of cuprous oxide/silver (Cu2O/Ag) hybrid as surface-enhanced Raman scattering probe for trace determination of methyl orange

Recently, there have been publications on preparing hybrid materials between noble metal and semiconductor for applications in surface-enhanced Raman scattering (SERS) substrates to detect some toxic organic dyes. However, the use of cuprous oxide/silver (Cu₂O/Ag) to measure the trace amounts of methyl orange (MO) has not been reported. Therefore, in this study, the trace level of MO in water solvent was determined using a SERS substrate based on Cu₂O microcubes combined with silver nanoparticles (Ag NPs). Herein, a series of Cu₂O/Agx (x= 1-5) hybrids with various Ag amounts was synthesized via a solvothermal method followed by a reduction process, and their SERS performance was studied in detail. X-ray diffraction (XRD) and scanning electron microscopy results confirmed that 10 nm Ag NPs were well dispersed on 200-500 nm Cu₂O microcubes to form Cu₂O/Ag heterojunctions. Using the as-prepared Cu₂O and Cu₂O/Agx as MO probe, the Cu₂O/Ag5 nanocomposite showed the highest SERS activity of all samples with the limit of detection as low to 1 nM and the enhancement factor as high as 4 × 10⁸. The logarithm of the SERS peak intensity at 1389 cm^-1 increased linearly with the logarithm of the concentration of MO in the range from 1 nM to 0.1 mM.

Sunlight-Driven Photocatalytic Degradation of Methylene Blue with Facile One-Step Synthesized Cu-Cu2O-Cu3N Nanoparticle Mixtures

Sunlight-driven photocatalytic degradation is an effective and eco-friendly technology for the removal of organic pollutants from contaminated water. Herein, we describe the one-step synthesis of Cu-Cu₂O-Cu₃N nanoparticle mixtures using a novel non-aqueous, sol-gel route and their application in the solar-driven photocatalytic degradation of methylene blue. The crystalline structure and morphology were investigated with XRD, SEM and TEM. The optical properties of the as-prepared photocatalysts were investigated with Raman, FTIR, UV-Vis and photoluminescence spectroscopies. The influence of the phase proportions of Cu, Cu₂O and Cu₃N in the nanoparticle mixtures on the photocatalytic activity was also investigated. Overall, the sample containing the highest quantity of Cu₃N exhibits the highest photocatalytic degradation efficiency (95%). This enhancement is attributed to factors such as absorption range broadening, increased specific surface of the photocatalysts and the downward band bending in the p-type semiconductors, i.e., Cu₃N and Cu₂O. Two different catalytic dosages were studied, i.e., 5 mg and 10 mg. The higher catalytic dosage exhibited lower photocatalytic degradation efficiency owing to the increase in the turbidity of the solution.

A high-performance SERS imprinted membrane based on Ag/CNTs for selective detection of spiramycin

In this test, the eggshell membrane (ESM) is selected as the support membrane for the biocompatibility and anchors CNTs on the surface to increase the mechanical properties. Then Ag NPs are decorated on CNTs-ESM substrate as SERS substrate by twice in-situ reduction. Finally, a layer of imprinted polymers is coated on the surface of the substrate to synthesize the imprinted membrane for selective detection of spiramycin. It is exhibited from the characteristic results that the CNTs significantly increase the mechanical properties and the detection sensitivity, simultaneously. When the concentration of SP changes between 10^-6 ∼ 10^-11 M, there is a linear relationship between SERS intensity and SP concentration. The detection limit is 10^-11 M, and the correlation coefficient R² is 0.9864. The SERS imprinted membrane can be applied into the detection of antibiotics in practical sample, which broadens the research field of antibiotics detection.

Multifunctional magnetic Fe3O4/Cu2O-Ag nanocomposites with high sensitivity for SERS detection and efficient visible light-driven photocatalytic degradation of polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) with carcinogenic, teratogenic and mutagenic properties are persistent organic pollutants in the environment. Herein, the novel multifunctional Fe₃O₄/Cu₂O-Ag nanocomposites (NCs) have been established for ultra-sensitive surface-enhanced Raman scattering (SERS) detection and visible light-driven photocatalytic degradation of PAHs. Fe₃O₄/Cu₂O-Ag NCs with different amounts of Ag nanocrystals were synthesized, and the effect of Ag contents on SERS performance was studied by finite-difference time-domain (FDTD) algorithm. The synergistic interplay of electromagnetic and chemical enhancement was responsible for excellent SERS sensitivity of Fe₃O₄/Cu₂O-Ag NCs. The limit of detection (LOD) of optimal SERS substrates (FCA-2 NCs) for Nap, BaP, Pyr and Ant was as low as 10^-9, 10^-9, 10^-9 and 10^-10 M, respectively. The SERS detection of PAHs in actual soil environment was also studied. Moreover, a simple SERS method was used to monitor the photocatalytic process of PAHs. The recovery and reuse of Fe₃O₄/Cu₂O-Ag NCs were achieved through magnetic field, and the outstanding SERS and photocatalytic performance were still maintained even after eight cycles. This magnetic multifunctional NCs provide a unique idea for the integration of ultra-sensitive SERS detection and efficient photocatalytic degradation of PAHs, and thus will have more hopeful prospects in the field of environmental protection.

Facile synthesis of nanocellulose-based Cu2O/Ag heterostructure as a surface-enhanced Raman scattering substrate for trace dye detection

Semiconductor metal-oxide/metal heterostructures with synergetic properties have potential applications in photocatalysis and optical sensors. Here, Cu₂O sub-micro cubes were synthesized under environmentally benign conditions using 2, 2, 6, 6-tetramethylpyperdine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils as a reducing and stabilizing agent. Then the surface of the Cu₂O cubes was decorated with silver nanoparticles (AgNPs) by a substitution reaction. The Cu₂O/Ag heterostructures within the cellulose nanofibrils (CNFs) network were employed as a promising surface-enhanced Raman scattering (SERS) assay for efficient sensing of methylene blue (MB), reaching a maximum enhancement factor (EF) of 4.0 × 10⁴. Their SERS intensities depended on the coverage density of AgNPs and the wavelength of the excitation laser. The excellent SERS performance may result from the charge transfer between Ag and Cu₂O molecules and the strong electromagnetic field at the interface. The CNF-Cu₂O/Ag substrates were capable of detecting MB dye down to 10^-8 M level with a relative standard deviation of 10-15%, demonstrating great sensitivity and reproducibility.

Enzyme-Free Molecularly Imprinted and Graphene-Functionalized Photoelectrochemical Sensor Platform for Pollutants

In this work, a label-free nonenzymatic photoelectrochemical (PEC) sensor is successfully developed for the detection of a typical pollutant, microcystin-LR (MC-LR), based on a visible-light-responsive alloy oxide, with highly ordered and vertically aligned Ti-Fe-O nanotubes (NTs) as substrates. Ti-Fe-O NTs consisting mainly of TiO₂ and atomically doped Fe₂O₃ are in situ prepared on a Ti-Fe alloy by electrochemical anodic oxidation. Using a simple electrochemical deposition technique, reduced graphene oxide (RGO) could be grown onto Ti-Fe-O NTs, exhibiting significant bifunctions. It not only provides an ideal microenvironment for functionalization of molecularly imprinted polymers (MIPs) on the surface but also serves as the PEC signal amplification element because of its outstanding conductivity for photons and electrons. The designed MIP/RGO/Ti-Fe-O NT PEC sensor exhibits high sensitivity toward MC-LR with a limit of detection as low as 10 pM. High selectivity toward MC-LR is also proven for the sensor. A promising detection platform not only for MC-LR but also for other pollutants has therefore been provided.

Self-Cleaning-Mediated SERS Chip Coupled Chemometric Algorithms for Detection and Photocatalytic Degradation of Pesticides in Food

Pesticide residues in food have been a grave concern to consumers. Herein, we have developed a dual-mode SERS chip using Cu₂O mesoporous spheres decorated with Ag nanoparticles (MCu₂O@Ag NPs) as both sensing and degradation/clearing unit for rapid detection of pymetrozine and thiram pesticides in tea samples. Three kinds of chemometric algorithms were comparatively applied to analyze the collected SERS spectra of pesticides. In comparison, random frog-partial least squares achieved the best performance with root mean square error of prediction and residual predictive deviation values of 0.9871, 6.17, and 0.9873, 6.64 for pymetrozine and thiram, respectively. Additionally, the prepared SERS chip showed great photocatalytic activity to degrade pesticides under visible light irradiation. Through a facile method, this work presented a novel dual-functional SERS chip for the rapid detection and degradation of low-concentration pesticides in both environmental and food samples.

Applications of surface-enhanced Raman spectroscopy in detection fields

Surface-enhanced Raman spectroscopy (SERS) is a Raman spectroscopy technique that has been widely used in food safety, environmental monitoring, medical diagnosis and treatment and drug monitoring because of its high selectivity, sensitivity, rapidness, simplicity and specificity in identifying molecular structures. This review introduces the detection mechanism of SERS and summarizes the most recent progress concerning the use of SERS for the detection and characterization of molecules, providing references for the later research of SERS in detection fields.

Surface Imprinted Layer of Cypermethrin upon Au Nanoparticle as a Specific and Selective Coating for the Detection of Template Pesticide Molecules

The detection of specific pesticides on food products is essential as these substances pose health risks due to their toxicity. The use of surface-enhanced Raman spectroscopy (SERS) takes advantage of the straightforward technique to obtain fingerprint spectra of target analytes. In this study, SERS-active substrates are made using Au nanoparticles (NPs) coated with a layer of polymer and followed by imprinting with a pesticide–Cypermethrin, as a molecularly imprinted polymer (MIP). Cypermethrin was eventually removed and formed as template cavities, then denoted as Au NP/MIP, to capture the analogous molecules. The captured molecules situated in-between the areas of high electromagnetic field formed by plasmonic Au NPs result in an effect of SERS. The formation of Au NP/MIP was, respectively, studied through morphological analysis using transmission electron microscopy (TEM) and compositional analysis using X-ray photoelectron spectroscopy (XPS). Two relatively similar pesticides, Cypermethrin and Permethrin, were used as analytes. The results showed that Au NP/MIP was competent to detect both similar molecules despite the imprint being made only by Cypermethrin. Nevertheless, Au NP/MIP has a limited number of imprinted cavities that result in sensing only low concentrations of a pesticide solution. Au NP/MIP is thus a specific design for detecting analogous molecules similar to its template structure.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.