Flexible and Reusable Ag Coated TiO2 Nanotube Arrays for Highly Sensitive SERS Detection of Formaldehyde

Advancements in reusable SERS substrates for trace analysis applications

Surface Enhanced Raman Spectroscopy (SERS) technique is an effective analytical technique in which fingerprint information about analytes can be obtained, can provide detection limit performance at the single molecule level, and analyzes are performed in a single step without any intermediate steps. SERS technique offers additional benefits rather than other analytical techniques including high selectivity, ultrasensitive detection, uncomplicated protocols, in situ sampling, on-set capability and cost-effectiveness. As a result of the combination of developments in materials and nanotechnology science with the SERS analysis technique, this technique strengthens its use advantage day by day. The most important factor that limited the use of this technique was the fact that the solution containing the desired analyte(s) was dropped onto the SERS substrate and the same substrate could not be reused in subsequent analyses. To solve this problem, scientists have focused on developing reusable SERS substrates in recent years. In these studies, scientists basically used three SERS substrate cleaning applications (1) washing the SERS substrate with a suitable solvent that can elute the analyte from SERS surface after analysis, (2) cleaning the SERS substrate with catalytic degradation of analytes after analysis by modifying them with catalytic active materials and (3) Applying plasma cleaning procedure to SERS substrate after analysis and (4) applying adsorption and desorption procedure prior to SERS analysis. Herein, the aim of this review article is to evaluate the reusable SERS substrates-based methods based on their level of development and their potential to recycle. This review offers a coherent discussion on a wide range of sensing schemes employed in fabricating the SERS substrates. We utilized a critical approach in which elaborative examples were selected to highlight key shortcomings of various experimental configurations. In the same vein, there is a discussion of the advantages and limitations concerning the key instrumental advances and the expansion of the recent methods developed in this area.

Preparation and characterization of cellulose nanofibril/chitosan aerogels with high-adsorbability and sensitive indication for indoor free formaldehyde

The toxic volatile organic compounds (VOCs), especially formaldehyde (FA), released from decoration materials pose a great threat to human health. In this study, formaldehyde adsorption performance of the specially formulated nanocellulose/chitosan aerogel (CNFCA) was investigated in simulated atmosphere. The physicochemical property of the composite aerogel was characterized, which had a large specific surface area (153.67 m2/g), a rough surface and an ultra-thin and porous structure. The composite aerogel showed excellent adsorption capacity for the formaldehyde, its theoretical maximum adsorption capacity was as high as 83.89 mg/g, and the adsorption process was more in accordance with the pseudo-second-order kinetics. The chromogenic reaction between the 4-amino-3-benzo-5-mercapto-1,2,4-triazolium (AHMT) and CNFCA was found that the color of the composite aerogel was depended on the free formaldehyde concentration. Based on this phenomenon, a colorimetric card was proposed and built to detection the formaldehyde in the atmosphere. Moreover, the adsorption mechanism research was found that the CNFCA with a multilayer structure belonged to physicochemical complex adsorption.

Fabrication of Flexible Pyramid Array as SERS Substrate for Direct Sampling and Reproducible Detection

Rapid extraction and analysis of target molecules from irregular surfaces are in high demand in the field of on-site analysis. Herein, a flexible platform used for surface-enhanced Raman scattering (SERS) based on an ordered polymer pyramid structure with half-imbedded silver nanoparticles (AgNPs) was prepared to address this issue. The fabrication includes the following steps: (1) creating inverted pyramid arrays in silicon substrate, (2) preparing a layer of AgNPs on the surface of the inverted pyramids, and (3) obtaining a substrate with an ordered polymer pyramids array with half-imbedded AgNPs by the molding method. This flexible substrate is capable of rapid extraction via a simple and convenient "paste and peel off" method. In addition, the substrate exhibits great repeatability and good sensitivity thanks to the uniformity and larger surface area of the ordered pyramids. The density of "hot spots" (local electromagnetic field with high intensity) is increased on the structured surface. Semi-imbedding silver particles in the polymer pyramids makes "hot spots" robust on the substrate. In addition, the preprepared silicon template with the inverted pyramids can be reused, which greatly reduces the production cost. With this substrate, we successfully analyzed thiram molecules on the epidermis of apples, cucumbers, and oranges, and the detection limits are 2.4, 3, and 3 ng/cm2, respectively. These results demonstrate the great potential of the substrate for in situ analysis, which can provide reference for the design of ideal SERS substrates.

Self-Assembled Bifunctional Copper Hydroxide/Gold-Ordered Nanoarray Composites for Fast, Sensitive, and Recyclable SERS Detection of Hazardous Benzene Vapors

Volatile organic compounds (VOCs), particularly monoaromatic hydrocarbon compounds (MACHs), pose a potential risk to the atmospheric environment and human health. Therefore, the progressive development of efficient detection methodologies is a pertinent need, which is still a challenge at present. In this study, we present a rapid and sensitive method to detect trace amounts of MACHs using a bifunctional SERS composite substrate. We prepared an Au/SiO₂ enhanced layer and a porous Cu(OH)₂ adsorption layer via microfluidic-assisted gas-liquid interface self-assembly. The composite substrate effectively monitored changes in benzaldehyde using time-varying SERS spectra, and track-specifically identified various VOCs such as benzene, xylene, styrene, and nitrobenzene. In general, the substrate exhibited a rapid response time of 20 s to gaseous benzaldehyde, with a minimum detection concentration of less than 500 ppt. Further experimental assessments revealed an optimum Cu(OH)₂ thickness of the surrounding adsorption layer of 150 nm, which can achieve an efficient SERS response to MACHs. Furthermore, the recoverable and reusable property of the composite substrate highlights its practicality. This study presents a straightforward and efficient approach for detecting trace gaseous VOCs using SERS, with significant implications in the designing of SERS substrates for detecting other VOCs.

Facile bimetallic co-amplified electrochemical sensor for folic acid sensing based on CoNPs and CuNPs

Folic acid (FA) is essential for human health, particularly for pregnant women and infants. In this work, a glassy carbon electrode (GCE) was modified by a bimetallic layer of Cu/Co nanoparticles (CuNPs/CoNPs) as a synergistic amplification element by simple step-by-step electrodeposition, and was used for sensitive detection of FA. The proposed CuNPs/CoNPs/GCE sensor was characterized by differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS) and field emission scanning electron microscopy (FESEM). Then, under optimal conditions, a linear relationship was obtained in the wide range of 110.00-1750.00 μM for the detection of FA with a limit of detection (LOD) of 34.79 μM (S/N = 3). The sensitivity was calculated as 0.096 μA μM^-1 cm^-2. Some interfering compounds including glucose (Glc), biotin, dopamine (DA), and glutamic acid (Glu) showed little effect on the detection of FA by amperometry (i-t). Finally, the average recovery obtained was in a range of 91.77-110.06%, with a relative standard deviation (RSD) less than 8.00% in FA tablets, indicating that the proposed sensor can accurately and effectively detect the FA content in FA tablets.

Ag Nanoparticles Decorated ZnO Nanorods as Multifunctional SERS Substrates for Ultrasensitive Detection and Catalytic Degradation of Rhodamine B

Industrial wastewater containing large amounts of organic pollutants is a severe threat to the environment and human health. Thus, the rapid detection and removal of these pollutants from wastewater are essential to protect public health and the ecological environment. In this study, a multifunctional and reusable surface-enhanced Raman scattering (SERS) substrate by growing Ag nanoparticles (NPs) on ZnO nanorods (NRs) was produced for detecting and degrading Rhodamine B (RhB) dye. The ZnO/Ag substrate exhibited excellent sensitivity, and the limit of detection (LOD) for RhB was as low as 10⁻¹¹ M. Furthermore, the SERS substrate could efficiently degrade RhB, with a degradation efficiency of nearly 100% within 150 min. Moreover, it retained good SERS activity after multiple repeated uses. The interaction between Ag NPs, ZnO, and RhB was further investigated, and the mechanism of SERS and photocatalysis was proposed. The as-prepared ZnO/Ag composite structure could be highly applicable as a multifunctional SERS substrate for the rapid detection and photocatalytic degradation of trace amounts of organic pollutants in water.

Process monitoring of photocatalytic degradation of 2,4-dinitrotoluene by Au-decorated Fe3O4@TiO2 nanoparticles: surface-enhanced Raman scattering method

Recently, the degradation and detection of 2,4-dinitrotoluene (2,4-DNT) capable of producing 2,4,6-trinitrotoluene (TNT) for environmental and human health risks have been developed. We prepared photoresponsive Au-decorated Fe₃O₄@TiO₂ nanoparticles (Fe₃O₄@TiO₂-Au NPs) under sunlight simulated Xe lamp irradiation. The photodegradation process of 2,4-DNT by Fe₃O₄@TiO₂-Au NPs was successfully monitored by surface-enhanced Raman scattering (SERS). Since SERS monitoring shows intrinsic information about the molecular structure, it was possible to predict the photodegradation of 2,4-DNT. The 2,4-DNT photodegradation mechanism based on two-dimensional correlation spectroscopy (2D-COS), which provides very beneficial information for a deeper understanding of systems, has been identified. We confirmed that Fe₃O₄@TiO₂-Au NPs can be widely used in organic pollutant degradation under sunlight. Furthermore, the combination of SERS based process monitoring and 2D-COS can be a convincing analytical technique for photodegradation studies of organic pollutants.

Facile preparation of MnO2-TiO2 nanotube arrays composite electrode for electrochemical detection of hydrogen peroxide

The MnO₂-TNTA composite electrodes were obtained through depositing MnO₂ into TiO₂ nanotube arrays (TNTA) by successive ionic layer adsorption reaction (SILAR) and subsequent hydrothermal method. The MnO₂-TNTA nanocomposites were used as electrochemical sensors for the detection of hydrogen peroxide (H₂O₂). The preparation conditions of MnO₂-TNTA electrodes and test conditions affect the electrochemical detection performance significantly. The optimal conditions are listed as follows: the number of SILAR cycles, 6 times; KMnO₄ solution temperature, 50 °C; supporting electrolyte, 0.5 M NaOH. Under these conditions, the MnO₂-TNTA electrode exhibits the best performance for detecting H₂O₂. The optimized MnO₂-TNTA electrode has a minimum detection limit of 0.6 μM (S/N = 3) and a linear range of 5 μM ∼ 13 mM, which is much superior to the previously-reported electrodes. Moreover, the optimized MnO₂-TNTA electrode possesses high selectivity, excellent stability and good reproducibility in the detection of H₂O₂. When used in the determination of H₂O₂ content in actual samples including disinfectant and milk, it also shows good accuracy, ideal recovery (96.00% ∼ 102.67%) and high precision (RSD < 4.0%).

Plasmonic Spherical Nanoparticles Coupled with Titania Nanotube Arrays Prepared by Anodization as Substrates for Surface-Enhanced Raman Spectroscopy Applications: A Review

As surface-enhanced Raman spectroscopy (SERS) continues developing to be a powerful analytical tool for several probes, four important aspects to make it more accessible have to be addressed: low-cost, reproducibility, high sensibility, and recyclability. Titanium dioxide nanotubes (TiO₂ NTs) prepared by anodization have attracted interest in this field because they can be used as safe solid supports to deposit metal nanoparticles to build SERS substrate nanoplatforms that meet these four desired aspects. TiO₂ NTs can be easily prepared and, by varying different synthesis parameters, their dimensions and specific features of their morphology can be tuned allowing them to support metal nanoparticles of different sizes that can achieve a regular dispersion on their surface promoting high enhancement factors (EF) and reproducibility. Besides, the TiO₂ photocatalytic properties enable the substrate's self-cleaning property for recyclability. In this review, we discuss the different methodological strategies that have been tested to achieve a high performance of the SERS substrates based on TiO₂ NTs as solid support for the three main noble metal nanoparticles mainly studied for this purpose: Ag, Au, and Pt.

Rapid field trace detection of pesticide residue in food based on surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy is an alternative detection tool for monitoring food security. However, there is still a lack of a conclusion of SERS detection with respect to pesticides and real sample analysis, and the summary of intelligent algorithms in SERS is also a blank. In this review, a comprehensive report of pesticides detection using SERS technology is given. The SERS detection characteristics of different types of pesticides and the influence of substrate on inspection are discussed and compared by the typical ways of classification. The key points, including the progress in real sample analysis and Raman data processing methods with intelligent algorithm, are highlighted. Lastly, major challenges and future research trends of SERS analysis of pesticide residue are also addressed. SERS has been proven to be a powerful technique for rapid test of residue pesticides in complex food matrices, but there still is a tremendous development space for future research.

TiO2hierarchical nano blooming-flower decorated by Pt for formaldehyde detection

To achieve an ultra-low concentration formaldehyde detection at low temperature, a platinum (Pt) assisted TiO₂hierarchical nano blooming-flower sphere material is synthesized through hydrothermal method. SEM and transmission electron microscope characterizations show that the diameter of the nano sphere was around 2μm with dissilient rods of 60 nm in diameter and 1μm in length on the surface. The response (R_a/R_g) achieved form this nanomaterial to HCHO is 1.08 (100 ppb) and 5.82 (5 ppm) at 130 °C without an involvement of any light source or solution. The relationship curve between the responses and concentrations shows regular exponential trend. The verification of sensor stability done by a 3 month reliability test shows no response-degradation. The optimal response and stability is attributed to the massive dissilient rods on the surface of TiO₂spheres and the assistance of Pt as a catalyzer disperses to intensify the formation of depletion area on the surface of TiO₂. This study provide an attractive and cost effective solution for the detection of HCHO in air at a relatively low temperature.

Plasma-Enabled Amorphous TiO2 Nanotubes as Hydrophobic Support for Molecular Sensing by SERS

We devise a unique heteronanostructure array to overcome a persistent issue of simultaneously utilizing the surface-enhanced Raman scattering, inexpensive, Earth-abundant materials, large surface areas, and multifunctionality to demonstrate near single-molecule detection. Room-temperature plasma-enhanced chemical vapor deposition and thermal evaporation provide high-density arrays of vertical TiO₂ nanotubes decorated with Ag nanoparticles. The role of the TiO₂ nanotubes is 3-fold: (i) providing a high surface area for the homogeneous distribution of supported Ag nanoparticles, (ii) increasing the water contact angle to achieve superhydrophobic limits, and (iii) enhancing the Raman signal by synergizing the localized electromagnetic field enhancement (Ag plasmons) and charge transfer chemical enhancement mechanisms (amorphous TiO₂) and by increasing the light scattering because of the formation of vertically aligned nanoarchitectures. As a result, we reach a Raman enhancement factor of up to 9.4 × 10⁷, satisfying the key practical device requirements. The enhancement mechanism is optimized through the interplay of the optimum microstructure, nanotube/shell thickness, Ag nanoparticles size distribution, and density. Vertically aligned amorphous TiO₂ nanotubes decorated with Ag nanoparticles with a mean diameter of 10-12 nm provide enough sensitivity for near-instant concentration analysis with an ultralow few-molecule detection limit of 10^-12 M (Rh6G in water) and the possibility to scale up device fabrication.

Manipulation and Applications of Hotspots in Nanostructured Surfaces and Thin Films

The synthesis of nanostructured surfaces and thin films has potential applications in the field of plasmonics, including plasmon sensors, plasmon-enhanced molecular spectroscopy (PEMS), plasmon-mediated chemical reactions (PMCRs), and so on. In this article, we review various nanostructured surfaces and thin films obtained by the combination of nanosphere lithography (NSL) and physical vapor deposition. Plasmonic nanostructured surfaces and thin films can be fabricated by controlling the deposition process, etching time, transfer, fabrication routes, and their combination steps, which manipulate the formation, distribution, and evolution of hotspots. Based on these hotspots, PEMS and PMCRs can be achieved. This is especially significant for the early diagnosis of hepatocellular carcinoma (HCC) based on surface-enhanced Raman scattering (SERS) and controlling the growth locations of Ag nanoparticles (AgNPs) in nanostructured surfaces and thin films, which is expected to enhance the optical and sensing performance.