Shintaro Pang

Development of a single aptamer-based surface enhanced Raman scattering method for rapid detection of multiple pesticides

The objective of this study was to develop a simple and rapid method that could detect and discriminate four specific pesticides (isocarbophos, omethoate, phorate, and profenofos) using a single aptamer-based capture procedure followed by Surface Enhanced Raman Spectroscopy (SERS). The aptamer is a single stranded DNA sequence that is specific to capture these four pesticides. The thiolated aptamer was conjugated onto silver (Ag) dendrites, a nanostructure that can enhance the Raman fingerprint of pesticides, through Ag-thiol bonds. It was then backfilled with 6-mercaptohexanol (MH) to prevent nonspecific binding. The modified SERS platform [Ag-(Ap + MH)] was then mixed with each pesticide solution (P) for 20 min. After capturing the pesticides, the Ag-(Ap + MH)-P complex was analyzed under a DXR Raman microscope and TQ Analyst software. The results show that the four pesticides can be captured and detected using principal component analysis based on their distinct fingerprint Raman peaks. The limits of detection (LODs) of isocarbophos, omethoate, phorate, and profenofos were 3.4 μM (1 ppm), 24 μM (5 ppm), 0.4 μM (0.1 ppm), and 14 μM (5 ppm) respectively. This method was also validated successfully in apple juice. These results demonstrated the super capacity of aptamer-based SERS in rapid detection and discrimination of multi-pesticides. This technique can be extended to detect a wide range of pesticides using specific aptamers.

Rapid Detection of Acetamiprid in Foods using Surface‐Enhanced Raman Spectroscopy (SERS)

Acetamiprid is a neonicotinoid pesticide that is commonly used in modern farming. Acetamiprid residue in food commodities can be a potential harm to human and has been implicated in the honey bee hive die off crisis. In this study, we developed rapid, simple, and sensitive methods to detect acetamiprid in apple juice and on apple surfaces using surface-enhanced Raman spectroscopy (SERS). No pretreatment of apple juice sample was performed. A simple surface swab method was used to recover acetamiprid from the apple surface. Samples were incubated with silver dendrites for several minutes and SERS spectra were taken directly from the silver surface. Detection of a set of 5 apple juice samples can be done within 10 min. The swab-SERS method took 15 min for a set of 5 samples. Resulting spectral data were analyzed using principal component analysis. The highest acetamiprid peak at 634 cm(-1) was used to detect and quantify the amount of acetamiprid spiked in 1:1 water-methanol solvent, apple juice, and on apple surface. The SERS method was able to successfully detect acetamiprid at 0.5 μg/mL (0.5 ppm) in solvent, 3 μg/mL (3 ppm) in apple juice, and 0.125 μg/cm(2) on apple surfaces. The SERS methods provide simple, rapid, and sensitive ways to detect acetamiprid in beverages and on the surfaces of thick skinned fruits and vegetables.

Highly sensitive and selective detection of nitrite ions using Fe3O4@SiO2/Au magnetic nanoparticles by surface-enhanced Raman spectroscopy.

A novel and pragmatic method was developed to detect the concentration of nitrite ions using Fe3O4@SiO2/Au magnetic nanoparticles (MNPs) by surface-enhanced Raman scattering (SERS). The as-prepared bifunctional nanocomposites can be used to simultaneously purify target molecules using external magnetic field and produce Raman fingerprint spectrum with trace level of target molecules. In acidic media, 4-aminothiophenol (4-ATP) molecules conjugated on Fe3O4@SiO2/Au MNPs were triggered by nitrite ions to form azo bonds, resulting in three new marker peaks on the SERS spectrum. Under optimized conditions, the detection limit based on the three marker peaks were 15.63, 13.69, and 17.77μM, which was much lower than the maximum NO2(-) concentration of 1.0mgL(-1) (71.4μM) allowed in drinking water as defined by U.S. Environmental Protection Agency (EPA). The specificity of this proposed method to detect nitrite ions was demonstrated using common ions as competitors. In addition, the SERS-based technique was successfully employed to detect nitrite ions in pond water, a synthetic urine solution, and pickle brine. Considering its good sensitivity and selectivity, the detection method is well suited for the detection of nitrite ions in real samples without pretreatment steps.

In situ SERS detection of multi-class insecticides on plant surfaces

Organophosphates, pyrethroids and neonicotinoids are three major classes of insecticides applied in tea, fruit and vegetable cultivation. Chromatographic techniques coupled with mass spectroscopy are commonly used for the detection and identification of these insecticides. However, time consuming sample extraction and clean-up are always needed for these techniques. The objective of this study was to develop an in situ surface enhanced Raman spectroscopic (SERS) method that could rapidly and sensitively detect and discriminate these multi-class pesticides on plant surfaces directly without extracting them out. Four insecticides including isocarbophos and phorate (both are organophosphates), deltamethrin (a pyrethroid) and imidacloprid (a neonicotinoid) were used to contaminate the surfaces of fresh tea leaves and apples. Then 4 μL 250 μg mL−1 gold nanoparticle colloids (50 nm, citrate coated) were deposited on the plant surface contaminated with pesticides and then air-dried for Raman measurements. The results show that the four pesticides can be detected and discriminated in situ using the developed SERS method. The limits of detection (LODs) of isocarbophos, phorate, imidacloprid and deltamethrin were 0.25, 0.25, 0.5 and 0.5 mg kg−1 on fresh tea leaves, and 0.01, 0.01, 0.02, 0.02 mg kg−1 on apple peels, respectively. The developed SERS method provides a simple, rapid, and sensitive way to monitor these insecticides on plant surfaces for safe production of commercial tea, fruits and vegetables.

Integration of colorimetric and SERS detection for rapid screening and validation of melamine in milk

Various analytical methods have been developed for detecting melamine in milk. Herein, we developed a novel method which integrated two gold nanoparticle (Au NP) based techniques, colorimetric and surface enhanced Raman spectroscopic (SERS) analyses, for rapid screening and validation of melamine in milk. The colorimetric method, which utilizes the color change of Au NPs from red to blue or purple upon interaction with melamine, was used for rapid screening. However, the colorimetric method presents false positive and inaccurate quantitative signals in the presence of interfering compounds. To overcome these limitations, the SERS method, which can directly utilize Au NPs from the colorimetric method, was employed as a rapid validation tool. In order to optimize the integration of these two methods, three sizes (15, 30, and 50 nm) of Au NPs were evaluated, and the 30 nm Au NPs were determined to be the best size for both colorimetric and SERS methods based on limits of detection (LODs) and quantification capability of melamine. By using the developed colorimetric–SERS method, we were able to rapidly screen and validate as low as 0.25 ppm melamine in milk within 20 min. Integrating colorimetric and SERS methods exploits the advantages of both methods, and provides a more rapid, accurate, and cost-effective way for monitoring melamine contamination in large amounts of food or feed products.

Evaluation of surface-enhanced Raman scattering detection using a handheld and a bench-top Raman spectrometer: A comparative study

Surface enhanced Raman scattering (SERS) detection using a handheld Raman spectrometer and a bench-top Raman spectrometer was systemically evaluated and compared in this study. Silver dendrites were used as the SERS substrate, and two pesticides, maneb and pyrrolidine dithiocarbamate-ammonium salt (PDCA) were used as the analytes. Capacity and performance were evaluated based on spectral resolution, signal variation, quantitative capacity, sensitivity, flexibility and intelligence for SERS detection. The results showed that the handheld Raman spectrometer had better data consistency, more accurate quantification capacity, as well as the capacity of on-site and intelligence for qualitative and semi-quantitative analysis. On the other hand, the bench-top Raman spectrometer showed about 10 times higher sensitivity, as well as flexibility for optimization of the SERS measurements under different parameters such as laser power output, collective time, and objective magnification. The study on the optimization of SERS measurements on a bench-top spectrometer provides a useful guide for designing a handheld Raman spectrometer, specifically for SERS detection. This evaluation can advance the application of a handheld Raman spectrometer for the on-site measurement of trace amounts of pesticides or other chemicals.

Alteration of the nonsystemic behavior of the pesticide ferbam on tea leaves by engineered gold nanoparticles

A model system consisting of a nonsystemic pesticide (ferbam), engineered gold nanoparticles (AuNPs) and a plant tissue (tea leaves) was investigated using surface enhanced Raman spectroscopy (SERS). Ferbam has no ability by itself to penetrate into tea leaves. When AuNPs were placed with ferbam onto the surface of tea leaves, however, the SERS signal of the ferbam-AuNPs complex was observed inside of the tea leaves. Within 1 h, the ferbam-AuNPs complex rapidly penetrated into the leaf to a depth of approximately 190 μm, about (1)/3 to (1)/2 of the leafs thickness. The rate of penetration was dependent on the size of AuNPs, with 30 nm AuNPs-ferbam penetrating more rapidly when compared with complexes made with the 50 and 69 nm AuNPs. These results clearly demonstrated an alteration of the nonsystemic behavior of ferbam in the combined presence with AuNPs. This finding might lead to the development of some new pesticide formulations. Conversely, new toxicity issues may arise as the behaviors and fate of pesticides are altered significantly upon interaction with engineered NPs in the pesticide formulation or environment.

Understanding the competitive interactions in aptamer–gold nanoparticle based colorimetric assays using surface enhanced Raman spectroscopy (SERS)

Aptamer–gold nanoparticle (AuNP) based colorimetric assays have become increasingly popular as viable rapid detection methods, but the molecular interactions governing the mechanism and successful interpretation of color changes have not been explored well. The objective of this study was to evaluate the competitive interactions that occur in this detection assay at the molecular level by employing surface enhanced Raman spectroscopy (SERS). The SERS signals of molecules in close proximity to AuNPs were exploited to give information on AuNP surface coverage as well as ssDNA aptamer conformational changes during target capture. Two antibiotics, ampicillin and kanamycin, and their respective aptamers were used in this study. The results indicate that the reason for the lack of AuNP aggregation with ampicillin could be due to a stronger binding affinity of AuNPs to ampicillin than to the aptamer. Kanamycin, on the other hand, induced AuNP aggregation to produce a color change and the SERS data indicate a stronger binding affinity of kanamycin to the aptamer than to the AuNPs as well as aptamer conformational changes. The use of SERS can be a potential tool to rapidly screen and validate the aptamer and target interaction for the application of aptamer–gold nanoparticle (AuNP) based colorimetric assays.

Innovative sandwich assay with dual optical and SERS sensing mechanisms for bacterial detection

The increased incidence of food pathogen outbreaks placed a new emphasis on the requirement of a rapid, sensitive, and reliable detection method for pathogens in food samples. Surface-enhanced Raman spectroscopy (SERS) is a technique that tremendously enhances the weak Raman scattering of an analyte by using a metallic nano-substrate. Herein, we developed an innovative SERS sandwich assay which is based on 3-mercaptophenylboronic acid (3-MPBA) as a capturer and reporter for the detection of bacteria. Using Salmonella enterica and Listeria monocytogenes as model bacteria, we have identified a unique bacterial SERS signal upon the interaction between the captured bacteria, 3-MBPA and silver nanoparticles (AgNPs), which was used as the base for reliable detection of bacteria using SERS mapping. Both optical and chemical (SERS mapping) imaging were used as mechanisms for bacterial detection and quantification. Our assay achieved sensitive and reliable detection of as low as 102 CFU mL−1 for Salmonella with great capability for quantification. The total analytical time for optical detection is 1.25 hours and for SERS imaging is 3 hours. Our assay represents an innovative platform for rapid, sensitive and reliable detection of total bacteria for an array of industrial applications.

Adsorption-enhanced hydrolysis of glucan oligomers into glucose over sulfonated three-dimensionally ordered mesoporous carbon catalysts

The hydrolysis of cellulose and β(1 → 4) oligosaccharides on carbon catalysts is a promising approach for the selective production of glucose from cellulose and its derivatives. In this study, a structure–property relationship was developed for the hydrolysis of water soluble glucan oligomers over three dimensionally ordered mesoporous (3DOm) carbon with sulfonic acid groups. It was found that the number of sulfonic acid groups on 3DOm carbon catalysts and their adsorption properties need to be optimized to maximize the hydrolysis of water soluble glucan oligomers. The adsorption capacity of glucan oligomers on 3DOm carbon increases with their chain length, which can be attributed to two molecular behaviors: (a) long glucan oligomers adsorbed on the carbon surface exhibit a dense packing structure compared with relatively small glucan oligomers; and (b) a part of a glucan oligomer “dangles” off the carbon surface due to its branched structure. It was found that the adsorption enthalpy and entropy increased with increasing the chain length of the glucan oligomers, which favors the hydrolysis reaction to produce glucose over the carbon catalysts.