Re-understanding of SERS for General and Standardized Quantitative Analysis.

We proposed two physical concepts, i.e., an intramolecular relative cross section (RCS) and an intermolecular relative scattering ability (RSA), to re-understand and re-describe surface-enhanced Raman scattering (SERS) and established a general SERS quantification theory. Interestingly, RCS and RSA are intrinsic factors and are experimentally measurable to form datasheets of molecules, namely, SERS cards, with which a standard SERS quantification procedure was established. The validity of the theory and quantification procedure was confirmed by experiments. Surprisingly, RCS and RSA are also valid for complex systems being considered as virtual molecules and are experimentally measurable. This simplifies complex systems into analyte-virtual molecule binary systems. With this consideration, trace-level mitoxantrone (a typical cancer drug metabolite) in artificial urine was accurately predicted. The theory, the SERS cards, the standard quantification procedure, and the virtual molecule concept pave a way toward quantitative and standardized SERS spectroscopy in dealing with real-world problems and complex samples.

Portable Copper-Based Electrochemical SERS Sensor for Point-of-Care Testing of Paraquat and Diquat by On-Site Electrostatic Preconcentration.

With the advent of portable Raman spectrometers, the deployment of surface-enhanced Raman spectroscopy (SERS) in point-of-care testing (POCT) has been initiated. Within any analytical framework employing SERS, the acuity and selectivity inherent to the SERS substrate are of paramount importance. In this article, we utilize in situ electrochemical passivation technology to fabricate CuI passivation film, which serves as a flexible copper-based SERS substrate. Furthermore, portable electrochemical SERS (EC-SERS) sensors were prepared by combining this with laser direct writing technology. The detection signal was amplified using electrostatic preconcentration technology, showcasing impressive sensitivity, selectivity, and stability in pesticide detection. The detected concentrations of paraquat and diquat in tea reached as low as 3.36 and 2.43 µg/kg, respectively. Furthermore, the application of electrostatic preconcentration facilitated selective target molecule aggregation on the SERS sensor, markedly increasing Raman signal strength and enabling single-molecule detection. This research introduces an innovative POCT method for pesticides, promising to advance environmental monitoring's analytical capabilities.

Highly Sensitive Detection of Formaldehyde by Laser-Induced Graphene-Coated Silver Nanoparticles Electrochemical Sensing Electrodes.

Formaldehyde (HCHO) poses a grave threat to human health because of its toxicity, but its accurate, sensitive, and rapid detection in aqueous solutions remains a major challenge. This study proposes a novel electrochemical sensor composed of a graphene-based electrode that is prepared via laser induction technology. The precursor material, polyimide, is modified via the metal ion exchange method, and the detective electrode is coated with graphene and silver nanoparticles. And the special structure of graphene-coated Ag was demonstrated using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) results show that graphene provides more sites for Ag NRs to be exposed and increases the surface area of contact between the solution and the detection object. In addition, differential pulse voltammetry (DPV) analysis exhibits high linearity over the HCHO concentration range from 0.05 to 5 µg/mL, with a detection limit of 0.011 µg/mL (S/N = 3). The Ag NPs in the electrochemical reaction will adsorb the intermediate •CO and •OH, catalyze their combination, and finally convert to CO2 and H2O, respectively. A microdetection device, specially designed for use with commercial micro-workstations, is employed to fully demonstrate the practical application of the electrode, which paves a way for developing formaldehyde electrochemical sensors.

Design of a versatile and selective electrochemical sensor based on dummy molecularly imprinted PEDOT/laser-induced graphene for nitroaromatic explosives detection.

Considering the formidable explosive power and human carcinogenicity of nitroaromatic explosives, the implementation of an accurate and sensitive detection technology is imperative for ensuring public safety and monitoring post-blast environmental contamination. In the present work, a versatile and selective electrochemical sensor based on dummy molecularly imprinted poly (3,4-ethylenedioxythiophene)/laser-induced graphene (MIPEDOT/LIG) was successfully developed and the specific detection of multiple nitroaromatic explosives was realized in the single sensor. The accessible and nontoxic trimesic acid (TMA) and superior 3, 4-ethylenedioxythiophene (EDOT) were selected as the dummy-template and the functional monomer, respectively. The interaction between the functional monomer and the template, and the morphology, electrochemical properties and detection performance of the sensor were comprehensively investigated by ultraviolet-visible spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry. Benefiting from the alliance of TMA and EDOT, the MIPEDOT/LIG sensor manifested outstanding selectivity and sensitivity for 2,4,6-trinitrotolueen (TNT), 2,4,6-trinitrophenol (TNP), 2,4-dinitrotoluene (DNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrophenol (DNP), and 1,3-dinitrobenzene (DNB) (representative nitroaromatic explosives) with limits of determination of 1.95 ppb, 3.06 ppb, 2.49 ppb, 1.67 ppb, 1.94 ppb, and 4.56 ppb, respectively. The sensor also exhibited extraordinary reliability and convenience for environmental sample detection. Therefore, a perfect combination of versatility and selectivity in the MIPEDOT/LIG sensor was achieved. The findings of this work provide a new direction for the development of multi-target electrochemical sensors using a versatile dummy template for explosives detection.

Slanted Ag-Al alloy nanorods arrays for highly active and stable surface-enhanced Raman scattering substrates.

Aluminum has been established as an earth-abundant and low-cost alternative to gold and silver for plasmonic applications. Particularly, aluminum largely tends to combines with oxygen compared with silver. Here, a simple glancing angle deposition technique is presented to prepare Ag-Al alloy nanorods (NRs) with a small amount of aluminum. The effect of aluminum is to combine oxygen or corroded substances under certain conditions, such as in the air and in etchants. Beside this, owing to the large diffusion coefficient of aluminum in a Si wafer, the aluminum diffuses easily into a Si wafer, so the bonding force between the Ag-Al alloy NRs and Si wafer can be improved accordingly. In this work, 3.5 at% Al alloy NRs are optimal to exhibit high surface-enhanced Raman scattering (SERS) sensitivity, long-time stability as well as strong bonding force with a Si wafer. Ag-Al alloy NRs make a metal-metal alloy a promising material platform to develop pretty sensitive as well as stable SERS substrates.

Development of a selective electrochemical microsensor based on molecularly imprinted polydopamine/ZIF-67/laser-induced graphene for point-of-care determination of 3-nitrotyrosine.

3-nitrotyrosine (3-NT) is a biomarker closely associated with the early diagnosis of oxidative stress-related disorders. The development of an accurate, cost-effective, point-of-care 3-NT sensor holds significant importance for self-monitoring and clinical treatment. In this study, a selective, sensitive, and portable molecularly imprinted electrochemical sensor was developed. ZIF-67 with strong adsorption capacity was facilely modified on an electrochemically active laser-induced graphene (LIG) substrate (formed ZIF-67/LIG). Subsequently, biocompatible dopamine was chosen as the functional monomer, and interference-free ʟ-tyrosine was used as the dummy template to create molecularly imprinted polydopamine (MIPDA) on the ZIF-67/LIG, endowing the sensor with selectivity. The morphologies, electrochemical properties, and detection performance of the sensor were comprehensively investigated using scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and differential pulse voltammetry. To achieve the best performance, several parameters were optimized, including the number of polymerization cycles (15), elution time (60 min), incubation time (7 min), and pH of the buffer solution (6). The turnaround time for this sensor is 10 min. Benefiting from the alliance of MIPDA, ZIF-67, and LIG, the sensor exhibited excellent sensitivity with a detection limit of 6.71 nM, and distinguished selectivity against 11 interfering substances. To enable convenient clinical diagnosis, a customized electrochemical microsensor with MIPDA/ZIF-67/LIG was designed, showcasing excellent reliability and convenience in detecting biological samples without pretreatment. The proposed microsensor will not only facilitate clinical diagnosis and improve patient care, but also provide inspiration for the development of other portable and accurate electrochemical biosensors.

A novel CO sensor based on the point contact between Pd decorated TiO2 nanotubes array.

To obtain a high sensitive CO sensor, a new nanostructure based on the point contact between Pd decorated TiO2 nanotubes was proposed in this paper. TiO2 nanotubes array was fabricated on titanium wire by electrochemical anodic oxidation, and Pd catalytic nanoparticles were modified by the micro-emulsion electrochemical deposition. The surface morphology was characterized by field emission scanning electron microscopy (FE-SEM), and CO sensitivity of the point contact between TiO2 nanotubes was investigated through the current-voltage (I-V) characteristic measurement. It was found that Pd epitaxially grew along the tube walls, and they were distributed on the surface of the nanotubes. The point contact based sensor between Pd decorated TiO2 nanotubes exhibited a strong temperature dependent sensitivity towards CO. From the I-V curves, we found the Schottky barrier was formed on the contact and the barrier height was also calculated.

H2 sensing properties of pd modified WO3-Fe2O3 nanostructured composite films prepared by amorphous W-Fe dealloying.

Tungsten trioxide nanopetal with Fe2O3 composite films were synthesized by dealloying the W-Fe alloy film in HCl aqueous solution; nano-Pd particles were directly electrochemical deposited on the surface of dealloying films in a micro-emulsion system and following thermal oxidation in air. The structure, morphology, chemical composition and quality of the prepared WO3 nanopetal with Fe2O3 composite films were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), respectively. The results showed that the thickness of the WO3 nanopetals were less than 50 nm, which were monoclinic phase after controlled thermal oxidation process, and the nano-Pd particles were evenly dispersed on the WO3 film surface with controlled diameters ranging from 20 to 40 nm. The sensors were tested for 25-2000 ppm H2 at temperatures from 50 to 200 degrees C; and the H2 sensing properties of Pd loading (1.54 at.%) WO3-Fe2O3 composited films exibited the best response at the low working temperature of 125 degrees C.

Preliminary investigation of solution diffusive behavior on V-doped TiO2 nanotubes array by electrochemical impedance spectroscopy.

V-doped TiO2 nanotubes array was successfully fabricated on a Ti-V alloy via an electrochemical anodization process. The crystal phase and surface morphology of the nanostructured film were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The solution diffusive behavior on TiO2 nanotubes was investigated by electrochemical impedance spectroscopy (EIS) analysis in an aqueous electrolyte containing 0.05 M Na2SO3. A schematic diagram of the interface solution induced into nanotube by photocatalysis on V-doped TiO2 under visible light irradiation was proposed in the study. Considerable photogenerated holes migrate to the interface of TiO2/electrolyte and react with OH-, forming hydroxyl radicals, which induce the electrolyte into the nanotube and improve the hydrophilicity. It was found that the photoelectrocatalytic reaction of TiO2 nanotubes determined the diffusion behavior of the solution; faster diffusion was observed on the V-doped TiO2 nanotubes array under visible light irradiation. The results also demonstrated that EIS is a powerful tool for characterizing the complicate diffusion behavior within the porous nanostructures.

A novel NO2 sensor based on TiO2 nanotubes array with in-situ Au decoration.

Au decorated TiO2 nanotubes array was successfully fabricated on a Ti-Au alloy via an electrochemical anodization process. The crystal phase and microstructure of the TiO2 nanotubes array were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), respectively. Au particles were well distributed among TiO2 nanotubes and acted as the catalyst. It was found that the sensor based on Ti-Au alloy anodization can be a promising sensor which was highly sensitive to low concentrations of nitrogen dioxide (NO2), and response at room temperature. The sensitivity of Au-decorated TiO2 nanotubes sensor was 36.11 to 8 ppm NO2 at room temperature. Meanwhile, the resistance signal of Au-decorated TiO2 sensor changes quickly within 80 s upon exposure to 8 ppm NO2 at 35 degrees C, the undecorated TiO2 however takes much more time (>110 s) to respond and the resistance signal remains unstable.

Robust quantitative SERS analysis with Relative Raman scattering intensities.

Robust quantitative analysis methods are very attractive but challenging with surface-enhanced Raman scattering (SERS) technique till now. Quantitative analysis methods using absolute Raman scattering intensities tend to desire very critical reproducibility of SERS substrates and consistency of testing conditions, as batch differences and inhomogeneity of SERS substrates as well as the fluctuation of measuring parameters placed challenging obstacles. Relative Raman scattering intensities, on the other hand, can release the adverse interferences mentioned above and provide effective and robust information as it is independent of the reproducibility of SERS substrates. By establishing external calibration working curves, we achieved accurate molecule composition prediction of molecules in multi-component systems. Further, by choosing or adding a label molecule with known concentration as Raman internal standards, the concentration of target molecules can be easily predicted. This approach proved the effectiveness and robustness of quantitative analysis with the relative Raman scattering intensities, even carried out with a flexible inhomogeneous SERS substrate.

Responses of human gingival fibroblasts to superhydrophilic hydrogenated titanium dioxide nanotubes.

Soft tissue integration is critical for the long-term retention of dental implants. The surface properties including topography and wettability can impact soft tissue sealing. In our work, a thermal hydrogenation technique was applied to modify anodized titanium dioxide nanotubes (TNTs). However, the effects of the hydrogenated surface on soft-tissue cells remain unclear. The aim of the present study was to investigate the bioactivities of human gingival fibroblasts (HGFs) on structured surfaces, which determine the early formation of soft tissue sealing. Three groups were examined: commercially pure titanium (Ti), anodized TNTs (air-TNTs) and hydrogenated TNTs (H2-TNTs). Scanning electron microscopy showed the nanotubular topography on the titanium surfaces after anodization. Then, hydrogenation ensured that the H2-TNTs were superhydrophilic with a contact angle of 3.5 ± 0.8°. In vitro studies such as cell adhesion assays, cell morphology, immunocytochemistry, wound healing assays, real-time PCR and enzyme-linked immunosorbent assays displayed enhanced adhesion, migration, relative gene expression levels, and extracellular matrix synthesis of the HGFs on H2-TNTs. Interestingly, focal adhesion kinase activation and integrin-mediated adhesion seemed to be induced by the H2-TNT surface. Our results revealed that a superhydrophilic nanostructure modified by anodization and hydrogenation can improve the bioactivity of HGFs and connective tissue regeneration, which will further promote and expand the application of titanium dioxide nanotubes in dental implants.

Enhanced hydrogen production on porous TiO2 nanotube arrays.

Well aligned TiO2 nanotube arrays have been synthesized via anodization in an NH4F and ethylene glycol electrolyte; the resulting carbon-entrained films were treated by oxygen and argon microwave plasma. It was found that as-prepared amorphous TiO2 nanotubes can be easily crystallized into anatase at temperature lower than 150 degrees C. Carbon can be effectively eliminated in oxygen plasma and a new secondary porosity was emerged. It was found such a porous film has obvious photovoltaic and hydrogen production enhancement under simulated solar irradiation compared with that crystallized in inert argon plasma. This phenomenon may be attributed to the improvement of light adsorption and its excellent capability of hole-electron separation derives from highly ordered nanoporous configurations.

Fabrication and hydrogen sensing properties of interlinked ribbon-like TiO2 films.

Interlinked ribbon-like TiO2 films were prepared by micro-arc oxidation (MAO) process and subsequent chemical-treatment of titanium substrate. The chemical-treatment included two steps: firstly, alkali treatment was performed on the surface of the porous TiO2 films, and then the samples were ion-exchanged in acid aqueous solution. The phase and microstructure of the samples were characterized by XRD, FE-SEM and TEM. It is found that ribbon-like sodium titanate is formed by alkali treatment, and its morphology remains unchanged after acid-treatment. However, the phase compositions of the samples surface change into TiO2 (MAOC-TiO2) after heat-treatment above 500 degrees C. The hydrogen sensing properties at low concentrations were investigated. The result shows that such ribbon-like TiO2 films present high sensing properties at a low temperature.

Hydrogen sensitivity performance of Pd doped SnO2 nanostructured film fabricated by spray pyrolysis.

The present investigation dealt with the fabrication of H2 gas sensor based on Pd doped SnO2. Porous SnO2 nanostructured film were fabricated by spray pyrolysis route using tin chloride pentahydrate (SnCl4.5H2O) solution as starting material, and PdCl2 as a dopant. Resistance measurements at different temperatures and concentration of H2 have been carried out with the samples. Microstructure and palladium dopant have been found to be critical factors determining the gas sensing properties of Pd/SnO2 specimens. The comparative gas sensitivity tests showed the excellent H2 sensing properties of the sensor in air was suggested to arise mainly from the improvement of gas absorption and catalytic effect of nano Pd dopant.

Preparation and visible light photocatalytic properties of (Er, La, N)-codoped TiO2 nanotube array films.

TiO2 nanotube array films were prepared by in-situ liquid phase transformation and deposition of anodic aluminum oxidation template films with (NH4)2TiF6 dilute solution; and the (Er, La, N)-codoped films were fabricated by impregnation with rare earth elements of Er and La following by heat treatment in flow ammonia. Obviously enhanced photocatalytic degradation of organic dye was obtained by codoped TiO2 under visible light irradiation compared with undoped TiO2 film, and the increase of photocatalytic performance of as-prepared TiO2 was attributed to the enlargement of light absorbency ranging from upconversed UV to red-shifted visible light up to 600 nm.

Using Atomic Force Microscopy to Measure Thickness of Passive Film on Stainless Steel Immersed in Aqueous Solution.

Employing atomic force microscopy (AFM) to measure passive film thickness on stainless steel (SS) in aqueous solution is proposed. SUS304 austenite and SUS329J4L duplex SS samples partly covered by gold were set in a minicell. To remove the original film, the SS surface but gold was etched using dilute sulfuric acid. After cleaning, open circuit potential (OCP), and distance from the sample surface to the top of the gold were measured. They were then immersed in either 1.0% NaCl; 5.0% NaCl; or aqueous solution with pH ranging from 1.0 to 10.0 and measured again. Differences between the first and subsequent measures of OCP suggested a passive film had formed in solution with pH ranging from 2.8 to 10.0. Similarly, differences among AFM measures revealed the observed film thickness increased with increase in pH and with decrease in chloride ions. Also, film thickness in water was greater than that in a vacuum. Comparison of AFM measurements of passive film on the austenite and sigma phases in sensitized SUS329J4L duplex SS revealed the film was thinner on the sigma phase containing more chromium. Taken together, these findings suggest the proposed method is applicable for measuring the thickness of passive films in aqueous solution.

Highly stable and active SERS substrates with Ag-Ti alloy nanorods.

Silver (Ag) nanostructures have been intensively studied as one of the most promising surface-enhanced Raman scattering (SERS) substrates; however, their practical applications have been limited by the chemical instability with regard to oxidation, sulfuration and etching of Ag. Therefore, designing and fabricating highly active Ag nanostructures with high SERS stability has been recognized as an important research area. Herein, Ag-Ti alloy nanorods (Ag-Ti alloy NRs) are designed and fabricated by the oblique angle deposition (OAD) method to protect Ag. Taking advantage of the higher chemical activity of Ti compared with Ag, Ti can be sacrificed against oxidation and corrosion, protecting Ag in harsh environments, further ensuring long-term stability of the SERS substrates. It is demonstrated that a 2% Ti (in atoms) substrate possesses extremely high SERS sensitivity, and is stable both in air for more than 1 month and in 10 mM HNO3 solution for 1 hour. The alloy nanostructure provides a new opportunity to achieve highly sensitive and highly stable SERS substrates.

Quantification of trace chemicals in unknown complex systems by SERS.

The chemical quantitative analysis at trace level has been widely explored by means of various techniques. While it still remains challenging to achieve ultrasensitive but facile, rapid, and inexpensive detection methods. In this paper, the possibility of employing surface-enhanced Raman scattering technique on a portable Raman system for rapid and accurate quantitative analysis of target chemicals in unknown systems was investigated. This detection approach contains 3 steps: (1) adding target chemicals with different amount to the initial unknown solution, leading to new solutions with target molecules of various concentrations; (2) Obtaining different samples' SERS spectra and capturing featured SERS peaks whose intensity grew up with the addition of target chemicals; (3) examining the relationship between featured peak intensity increment after adding target chemicals and its corresponding addition amount, and thus we could perform quantitative analysis of the chemical in an unknown solution and obtain its initial concentration. The validity of this method was systematically demonstrated by estimating the concentrations of 2-Naphthalenethiol (2-NaT) and 4-Mercaptopyridine (4-MPY) both in their single-component solutions and binary solutions, respectively. Predictions are close to their real values. Furthermore, we successfully predicted the concentrations of malachite green (MG) in fish water and benzidine in ground water. This study clearly demonstrates an accurate and facile approach to calculate the concentration of target chemicals in unknown systems, which fully exploit the potential of SERS quantitative analysis.

Effects of hydrogenated TiO2 nanotube arrays on protein adsorption and compatibility with osteoblast-like cells.

BACKGROUND: Modified titanium (Ti) substrates with titanium dioxide (TiO2) nanotubes have broad usage as implant surface treatments and as drug delivery systems. METHODS: To improve drug-loading capacity and accelerate bone integration with titanium, in this study, we hydrogenated anodized titanium dioxide nanotubes (TNTs) by a thermal treatment. Three groups were examined, namely: hydrogenated TNTs (H2-TNTs, test), unmodified TNTs (air-TNTs, control), and Ti substrates (Ti, control). RESULTS: Our results showed that oxygen vacancies were present in all the nanotubes. The quantity of -OH groups greatly increased after hydrogenation. Furthermore, the protein adsorption and loading capacity of the H2-TNTs were considerably enhanced as compared with the properties of the air-TNTs (P<0.05). Additionally, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate the interactions of TNTs with proteins. During the protein-loading process, the H2-TNTs not only enabled rapid protein adsorption, but also decreased the rate of protein elution compared with that of the air-TNTs. We found that the H2-TNTs exhibited better biocompatibility than the air-TNT and Ti groups. Both cell adhesion activity and alkaline phosphatase activity were significantly improved toward MG-63 human osteoblast-like cells as compared with the control groups (P<0.05). CONCLUSION: We conclude that hydrogenated TNTs could greatly improve the loading capacity of bioactive molecules and MG-63 cell proliferation.