A simple and sensitive SERS quantitative analysis method for urea using the dimethylglyoxime product as molecular probes in nanosilver sol substrate

Engineered aptamers for molecular imaging

Molecular imaging, including quantification and molecular interaction studies, plays a crucial role in visualizing and analysing molecular events occurring within cells or organisms, thus facilitating the understanding of biological processes. Moreover, molecular imaging offers promising applications for early disease diagnosis and therapeutic evaluation. Aptamers are oligonucleotides that can recognize targets with a high affinity and specificity by folding themselves into various three-dimensional structures, thus serving as ideal molecular recognition elements in molecular imaging. This review summarizes the commonly employed aptamers in molecular imaging and outlines the prevalent design approaches for their applications. Furthermore, it highlights the successful application of aptamers to a wide range of targets and imaging modalities. Finally, the review concludes with a forward-looking perspective on future advancements in aptamer-based molecular imaging.

Application of SERS-based nanobiosensors to metabolite biomarkers of CKD

A clinical diagnosis of chronic kidney disease (CKD) is commonly achieved by estimating the serum levels of urea and creatinine (CR). Given the limitations of the conventional diagnostic assays, it is imperative to seek alternative, economical strategies for the detection of CKD-specific biomarkers with high specificity and selectivity. In this respect, surface-enhanced Raman spectroscopy (SERS) can be regarded as an ideal choice. SERS signals can be greatly amplified by noble metal nanoparticles (e.g., gold nanoparticles (GNPs)) of numerous sizes, shapes, and configurations to help achieve ultra-sensitive single molecule-level detection at 10^-15 M (up to 10 orders of magnitude more sensitive than fluorescence-based detection). The irregular geometry of GNPs with spike-like tips, dimers, and aggregates with small nanogaps (i.e., due to plasmon coupling such as Raman hot spots) play a pivotal role in enhancing the specificity and sensitivity of SERS. This review critically outlines the performance of SERS-based biosensors in the ultrasensitive detection of CKD biomarkers in various body fluids in terms of basic quality assurance parameters (e.g., limit of detection, figure of merit, enhancement factor, and stability of the biosensor). Moreover, the challenges and perspectives are described with respect to the expansion of such sensing techniques in practical clinical settings.

Signaling strategies of silver nanoparticles in optical and electrochemical biosensors: considering their potential for the point-of-care

Silver nanoparticles (AgNPs) have long been overshadowed by gold NPs' success in sensor and point-of-care (POC) applications. However, their unique physical, (electro)chemical, and optical properties make them excellently suited for such use, as long as their inherent higher instability toward oxidation is controlled. Recent advances in this field provide novel strategies that demonstrate that the AgNPs' inherent capabilities improve sensor performance and enable the specific detection of analytes at low concentrations. We provide an overview of these advances by focusing on the nanosized Ag (in the range of 1-100 nm) properties with emphasis on optical and electrochemical biosensors. Furthermore, we critically assess their potential for point-of-care sensors discussing advantages as well as limitations for each detection technique. We can conclude that, indeed, strategies using AgNP are ready for sensitive POC applications; however, research focusing on the simplification of assay procedures is direly needed for AgNPs to make the successful jump into actual applications.

Progress on aptamer-based SERS sensors for food safety and quality assessment: methodology, current applications and future trends

It is well known that food safety has aroused extensive attentions from governments to researchers and to food industries. As a versatile technology based on molecular interactions, aptamer sensors which could specifically identify a wide range of food contaminants have been extensively studied in recent years. Surface-enhanced Raman spectroscopy integrated aptamer combines the advantages of both technologies, not only in the ability to specifically identify a wide range of food contaminants, but also in the ultra-high sensitivity, simplicity, portable and speed. To provide beneficial insights into the evaluation techniques in the field of food safety, we offer a comprehensive review on the design strategies for aptamer-SERS sensors in different scenarios, including non-nucleic acid amplification methods ("on/off" mode, sandwich mode, competition model and catalytic model) and nucleic acid amplification methods (hybridization chain reaction, rolling circle amplification, catalytic hairpin assembly). Meanwhile, a special attention is paid to the application of aptamer-SERS sensors in biological (foodborne pathogenic, bacteria and mycotoxins) and chemical contamination (drug residues, metal ions, and food additives) of food matrix. Finally, the challenges and prospects of developing reliable aptamer-SERS sensors for food safety were discussed, which are expected to offer a strong guidance for further development and extended applications.

Sensitive surface-enhanced Raman spectroscopy (SERS) determination of nitrofurazone by β-cyclodextrin-protected AuNPs/γ-Al2O3 nanoparticles

A novel surface-enhanced Raman spectroscopy (SERS) method for the determination of nitrofurazone was developed using AuNPs/γ-Al₂O₃ nanoparticles protected by β-cyclodextrin (β-CD) as substrate prepared in our lab. The optimum experimental conditions were obtained from single factor procedure and response surface modeling. A linear relationship (I_SERS = 508.96c + 31987.87, c: nmol L^-1, R² = 0.996) between SERS intensity and the concentration of nitrofurazone in the range of 3.3 - 667.0 nmol L^-1 was established, the limit of detection (LOD) was found at nmol L^-1 level (0.37 nmol L^-1 by 3S₀/S). The selectivity for the method was studied by the influences of foreign substances on the determination. The recoveries and RSD (n = 5) for the six meat samples were 95.1 % - 104.5% and 2.4 % - 4.8% respectively, which suggesting that the new SERS method was successfully to detecting nitrofurazone.

On-signal amplification of silver nanosol RRS/SERS aptamer detection of ultratrace urea by polystyrene nanosphere catalyst

The catalytic amplification signal of polystyrene nanosphere (PN) is used to conveniently fabricate the resonance Rayleigh scattering (RRS)/surface-enhanced Raman scattering (SERS) dual-mode method to sensitively and selectively detect urea in food. PN has strong catalysis of the slow nanoreaction of citrate-Ag(I) to produce yellow silver nanoparticles (AgNP), which exhibit strong RRS effect and SERS effect with molecular probes. When aptamer (Apt) is present, the Apt is adsorbed on the PN surface, the catalysis is weakened, the AgNP is reduced, and the SERS/RRS signal is weakened. After adding urea to exhibit specific Aptamer reaction, the Apt is desorbed from the PN surface and the catalysis is restored. As urea increase, the desorbed PNs increase to produce more AgNPs indicator to increase SERS/RRS signal. The increase value △I of SERS/RRS is linearly to urea concentration. Therefore, a sensitive and selective SERS/RRS dual-mode method for urea is established based on aptamers-regulated the catalysis of PNs. This method is applied to the detection of urea in milk with satisfactory results. The relative standard deviation is 3.9-6.8% and the recovery rate is 94.5-102%.

OUP accepted manuscript

A reversed-phase isocratic elution high-performance liquid chromatography method coupled with fluorescence detection has been developed to determine urea concentration via online postcolumn derivatization. Swimming pool water samples were filtered through 0.20 μm syringe filters. When the temperature of reaction coil was 40°C, urea was derivatized well with xanthydrol methanol solution (0.1 g/L) containing 0.50% hydrochloric acid with a flow rate of 0.20 mL/min. Successful separation was achieved by using Shim-pack VP-ODS C18 (250 mm × 4.6 mm, 5 μm) column, with a mobile phase containing phosphoric acid solution (0.01 mol/L) at a flow rate of 0.80 mL/min. Retention time and external standard method were used for qualitative and quantitative urea analysis, respectively. Under the established conditions, the limit of detection, linear range, correlation coefficient, recovery and relative standard deviation was 0.09 mg/L, 1.0-100.0 mg/L, 0.9998, 87.0-105.3% and 0.95-4.8%, respectively. Ammonia, thiourea and trichloroisocyanuric acid did not interfere with urea analysis. The method showed satisfactory results with high precision, accuracy, recovery, as well as sensitivity, for the determination of urea in swimming pool water.

Sensitive and reliable identification of fentanyl citrate in urine and serum using chloride ion-treated paper-based SERS substrate

Recently, the phenomenon of fentanyls overdose leading to death is emerging in an endless stream. There is an urgent requirement to quickly identify fentanyl content in body fluids for medical and judicial purposes. With this in mind, we present a paper-based SERS substrate decorated with uniform gold nanospheres treated by chloride ion for the detection of fentanyl citrate in urine and serum. In particular, the paper-based SERS sensor was prepared by liquid/liquid self-assembly technique and chloride ion was introduced to clean and modify the substrate surface, which improved the sensitivity of the solid substrate with an enhancement factor (EF) as high as 1.64 × 10⁵. Moreover, the uniformity of each paper-based substrate and the repeatability on different batches of substrate were excellent, and there was no obvious change in the intensity response of Raman spectra within a month. As a result, the quantitative analysis of fentanyl citrate in artificial urine and rat serum were performed based on the modified paper-based substrate with the limit of detection as low as 0.59 μg/mL and 2.78 μg/mL, respectively. Both the concentrations of the two biological samples with the Raman signal intensity were linearly plotted and the recovery of the spiked samples with different concentrations was collected to verify the accuracy of the quantitative curves. All the results suggest that this work makes SERS method available for the rapid identification and quantitative analysis of illicit drug in the real biological samples.

Aptamer Turn-On SERS/RRS/Fluorescence Tri-mode Platform for Ultra-trace Urea Determination Using Fe/N-Doped Carbon Dots

Sensitive and selective methods for the determination of urea in samples such as dairy products are important for quality control and health applications. Using ammonium ferric citrate as a precursor, Fe/N-codoped carbon dots (CD_FeN) were prepared by a hydrothermal procedure and characterized in detail. CD_FeN strongly catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂ to turn on an indicator molecular reaction, forming an oxidized tetramethylbenzidine (TMB_ox) probe with surface-enhanced Raman scattering, resonance Rayleigh scattering, and fluorescence (SERS, RRS, and FL) signals at 1,598 cm⁻¹, 370 nm, and 405 nm, respectively. The urea aptamer (Apt) can turn off the indicator reaction to reduce the tri-signals, and the addition of urea turns on the indicator reaction to linearly enhance the SERS/RRS/FL intensity. Thus, a novel Apt turn-on tri-mode method was developed for the assay determination of ultra-trace urea with high sensitivity, good selectivity, and accuracy. Trace adenosine triphosphate and estradiol can also be determined by the Apt-CD_FeN catalytic analytical platform.

Fabrication of large scale uniform copper-island thin film for ultrasensitive surface enhanced Raman scattering

Nanostructured metals with designable and controllable structures have received increasing attention in surface enhanced Raman scattering (SERS) due to the single molecular detection limit. Great challenges still remain in creating large scale substrates with high-density 'hotspots' to provide a uniform and stable enhancement of Raman signals. Here, we fabricated a copper island thin film over an 80 cm² scale substrate with tunable particle sizes by combining sputtering with dealloying processes. The island size can be tailored from 150 nm to 370 nm by controlling parameters and etching conditions and possesses an optimized surface morphology structure. The detection limit of crystal violet (CV) molecules reached 0.1 pM. Meanwhile, the copper island thin film presents good homogeneity and stability. Our method is promising to repeatedly fabricate novel metal SERS substrates on a large scale with standard properties for sensing applications.

Stimulus-responsive surface-enhanced Raman scattering: a "Trojan horse" strategy for precision molecular diagnosis of cancer

Molecular diagnosis has played an increasingly important role in cancer detection. However, it remains challenging to develop an in situ analytical method capable of profiling the molecular phenotype of tumors for precision cancer diagnosis. A "Trojan horse" strategy based on stimulus-responsive surface-enhanced Raman scattering (SR-SERS) is reported here for selectively recording the comprehensive molecular information of tumors in situ, without resorting to destructive sample preparation and complex data analysis. This technique is employed to delineate the margin between tumors and normal tissues with high accuracy, and to further discriminate the molecular fingerprints of tumors in the early and late stages. Based on molecular profiling, we discovered that the signal ratios of fatty acid-to-phenylalanine could serve as promising indicators for identifying the primary tumors in different stages. This simple SR-SERS technique also provides a potential useful means for identifying tumor classifications or distinguishing primary and metastatic tumors.

Mesoporous silica supported orderly-spaced gold nanoparticles SERS-based sensor for pesticides detection in food

In this study, a novel sensor fabricated with compactly arranged gold nanoparticles (AuNPs) templated from mesoporous silica film (MSF) via air-water interface has been confirmed as a promising surface-enhanced Raman scattering (SERS) substrate for detecting trace levels of 2,4-dichlorophenoxyacetic acid (2,4-D), pymetrozine and thiamethoxam. The densely arranged AuNPs@MSF had an average AuNPs size of 5.15 nm with small nanogaps (<2nm) between AuNPs, and exhibited a high SERS performance. SERS spectra of pesticides were collected after their adsorption on the AuNPs@MSF. The results showed that the concentration of 2,4-D, pymetrozine and thiamethoxam gave a good linear relationship with SERS intensity. Moreover, the designed SERS-based sensor (AuNPs@MSF) was stable for 3 months with ca. 3% relative standard deviation (RSD) and was applied successfully for the analysis of 2,4-D extraction from both environmental and food samples. The proposed SERS-based sensor was further validated by HPLC and showed satisfactory result (p > 0.05).

Multibranch Gold Nanoparticles as Surface-Enhanced Raman Spectroscopy Substrates for Rapid and Sensitive Analysis of Fipronil in Eggs

A sensitive strategy to rapidly detect fipronil residues in eggs using multibranch gold nanoparticles (AuNPs) as the substrate of surface-enhanced Raman spectroscopy (SERS) was investigated in this study. Under optimized conditions, fipronil molecules preferentially deposited on the multibranch gold nanoparticles with preferential (111) facet-oriented growth due to its low surface energy. This anisotropic growth promoted the increase of SERS "hot spots", inducing a huge enhancement of Raman signals of the fipronil. An external standard calibration method was employed for quantitative analysis, and the method was validated for linearity, sensitivity, repeatability and recovery. Good linearity were found in the concentration range of 10 ng/L~10 mg/L in fipronil acetone solution (R2 = 0.9916) and 8 × 10-5 mg/m2 to 0.8 mg/m2 on eggshells (R2 = 0.9906), respectively. The recovery rate based on acetone recovered fipronil on eggshells and in egg liquids was 80.13%~87.87%, and 81.34%~88.89%, respectively. The SERS assay was successfully used to monitor fipronil in eggs.

Control of Silver Coating on Raman Label Incorporated Gold Nanoparticles Assembled Silica Nanoparticles

Signal reproducibility in surface-enhanced Raman scattering (SERS) remains a challenge, limiting the scope of the quantitative applications of SERS. This drawback in quantitative SERS sensing can be overcome by incorporating internal standard chemicals between the core and shell structures of metal nanoparticles (NPs). Herein, we prepared a SERS-active core Raman labeling compound (RLC) shell material, based on Au⁻Ag NPs and assembled silica NPs (SiO₂@Au@RLC@Ag NPs). Three types of RLCs were used as candidates for internal standards, including 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP) and 4-methylbenzenethiol (4-MBT), and their effects on the deposition of a silver shell were investigated. The formation of the Ag shell was strongly dependent on the concentration of the silver ion. The negative charge of SiO₂@Au@RLCs facilitated the formation of an Ag shell. In various pH solutions, the size of the Ag NPs was larger at a low pH and smaller at a higher pH, due to a decrease in the reduction rate. The results provide a deeper understanding of features in silver deposition, to guide further research and development of a strong and reliable SERS probe based on SiO₂@Au@RLC@Ag NPs.

Hybrid synthetic receptor composed of molecularly imprinted polydopamine and aptamers for impedimetric biosensing of urea

An electrochemical aptamer-based method is described for highly specific sensing of urea. Urea-imprinted polydopamine was obtained by electropolymerization of dopamine (DA). The molecularly imprinted polymer (MIP) also contains DNA aptamers on gold nanoparticles decorated with a carbon nanotube network (AuNP/CNT). The material was placed on a glassy carbon electrode (GCE). After removal of urea from the MIP cavities, the GCE display double recognition capability which makes it superior to conventional MIP-only or aptamer-only based assays. On exposure of the modified electrode to urea, the interfacial charge transfer of the redox probe hexacyanoferrate is traced, typically measured at a peak voltage of 0.22 V vs. Ag/AgCl. The change in charge transfer resistance depends on the urea concentration. The assay has a 900 fM detection limit, and response is the linear up to 500 nM urea concentrations. Graphical abstract ᅟ.