Recent progress in the application of microfluidic systems and gold nanoparticles in immunoassays

A microfluidic chip-based multivalent DNA walker amplification biosensor for the simultaneous detection of multiple food-borne pathogens

The rapid, simultaneous, sensitive detection of the targets has important application prospects for disease diagnosis and biomedical studies. However, in practical applications, the content of the targets is usually very low, and signal amplification strategies are often needed to improve the detection sensitivity. DNAzyme-driven DNA walkers are an excellent signal amplification strategy due to their outstanding specificity and sensitivity. Food-borne pathogens have always been a foremost threat to human health, and it is an urgent demand to develop a simple, rapid, sensitive, and portable detection method for food-borne pathogens. In addition, there are various species of pathogens, and it is difficult to simultaneously detect multiple pathogens by a single DNA walker. For this reason, a substrate strand with three rA cleavage sites was cleverly designed, and a multivalent DNA walker sensor combined with the microfluidic chip technology was proposed for the simultaneous, rapid, sensitive analysis of Vibrio parahaemolyticus, Salmonella typhimurium, and Staphylococcus aureus. The developed sensor could be used to detect pathogens simultaneously and efficiently with low detection limits and wide detection ranges. Moreover, the combination of gold stirring rod enrichment and DNA walker achieved double amplification, which greatly improved the detection sensitivity. More importantly, by changing the design of the substrate chain, the sensor was expected to be used to detect other targets, thus broadening the scope of practical applications. Therefore, the sensor can build novel detection tool platforms in the field of biosensing.

Nanoparticle-based Point of Care Immunoassays for in vitro Biomedical Diagnostics

In resource-limited settings, the availability of medical practitioners and early diagnostic facilities are inadequate relative to the population size and disease burden. To address cost and delayed time issues in diagnostics, strip-based immunoassays, e.g. dipstick, lateral flow assay (LFA) and microfluidic paper-based analytical devices (microPADs), have emerged as promising alternatives to conventional diagnostic approaches. These assays rely on chromogenic agents to detect disease biomarkers. However, limited specificity and sensitivity have motivated scientists to improve the efficiency of these assays by conjugating chromogenic agents with nanoparticles for enhanced qualitative and quantitative output. Various nanomaterials, which include metallic, magnetic and luminescent nanoparticles, are being used in the fabrication of biosensors to detect and quantify biomolecules and disease biomarkers. This review discusses some of the principles and applications of such nanoparticle-based point of care biosensors in biomedical diagnosis.

Streptavidin-biotin-peroxidase nanocomplex-amplified microfluidics immunoassays for simultaneous detection of inflammatory biomarkers

Simultaneous, sensitive and quantitative detection of biomarkers in infectious disease is crucial for guiding antimicrobial treatment and predicting prognosis. This work reported an ultrasensitive and quantitative microfluidic immunoassay combined with the streptavidin-biotin-peroxidase (SA-B-HRP) nanocomplex-signal amplification system (MIS) to detect two inflammatory biomarkers, procalcitonin (PCT, for discriminating bacterial infections from nonbacterial infections) and interleukin-6 (IL-6, for monitoring the kinetics of infectious disease) simultaneously. The amplification system was based on the one step self-assembly of SA and B-HRP to form the SA-B-HRP nanocomplex, which effectively amplified the chemiluminescent signals. The linear ranges for PCT and IL-6 detections by MIS were 250-1.28 × 10⁵ pg mL^-1 and 5-1280 pg mL^-1, and the limit of detection (LOD) were 48.9 pg mL^-1 and 1.0 pg mL^-1, respectively, both of which were significantly improved compared with microfluidic immunoassays without amplification system (MI). More importantly, PCT and IL-6 in human serum could be simultaneously detected in the same run by MIS, which could greatly improve the detection efficiency and reduce the cost. Given the advantages of high sensitivity, multiplex and quantitative detection, MIS could be potentially applied for detection of biomarkers at low concentration in clinical diagnosis.

Detection of the nanomolar level of total Cr[(iii) and (vi)] by functionalized gold nanoparticles and a smartphone with the assistance of theoretical calculation models

We report a method for rapid, effective detection of both Cr(iii) and Cr(vi) (in the form of Cr(3+) and Cr2O7(2-), the main species of chromium in the natural environment) by making use of meso-2,3-dimercaptosuccinic acid (DMSA)-functionalized gold nanoparticles (Au NPs). The limit of detection (LOD) is 10 nM with the naked eye and the assay can be applied in detecting chromium in polluted soil from Yun-Nan Province in Southwest China. We use density functional theory to calculate the change of the Gibbs free energy (ΔG) of the interactions between the DMSA-Au NP system and various metal ions, which shows that DMSA-Au NPs have high specificity for both Cr(3+) and Cr2O7(2-).

An ultrasensitive, non-enzymatic glucose assay via gold nanorod-assisted generation of silver nanoparticles

This report demonstrates a colorimetric, non-enzymatic glucose assay with a low detection limit of 0.07 μM based on negatively charged gold nanorod-enhanced redox reaction. This glucose assay could generate silver nanoparticles as the readout that can be visualized by the naked eye, and only 4 femtomoles of nanorods are needed for glucose determination in one human plasma sample.

Integrated acoustic immunoaffinity-capture (IAI) platform for detection of PSA from whole blood samples

On-chip detection of low abundant protein biomarkers is of interest to enable point-of-care diagnostics. Using a simple form of integration, we have realized an integrated microfluidic platform for the detection of prostate specific antigen (PSA), directly in anti-coagulated whole blood. We combine acoustophoresis-based separation of plasma from undiluted whole blood with a miniaturized immunoassay system in a polymer manifold, demonstrating improved assay speed on our Integrated Acoustic Immunoaffinity-capture (IAI) platform. The IAI platform separates plasma from undiluted whole blood by means of acoustophoresis and provides cell free plasma of clinical quality at a rate of 10 uL/min for an online immunoaffinity-capture of PSA on a porous silicon antibody microarray. The whole blood input (hematocrit 38-40%) rate was 50 μl min(-1) giving a plasma volume fraction yield of ≈33%. PSA was immunoaffinity-captured directly from spiked female whole blood samples at clinically significant levels of 1.7-100 ng ml(-1) within 15 min and was subsequently detected via fluorescence readout, showing a linear response over the entire range with a coefficient of variation of 13%.

Label-free colorimetric sensing of cobalt(II) based on inducing aggregation of thiosulfate stabilized gold nanoparticles in the presence of ethylenediamine

As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co(2+) in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S(2)O(3)(2-)) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co(2+) first reacted with en and formed complexes of Co(en)(3)(2+) in aqueous solutions, which was followed by the oxidation of Co(en)(3)(2+) to Co(en)(3)(3+) by dissolved oxygen. Co(en)(3)(3+) then attacked S(2)O(3)(2-) ligands adsorbed on the AuNPs' surfaces, forming positively charged (en)(2)CoS(2)O(3)(+) on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S(2)O(3)(2-) and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD = 0.0 4 μM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu(2+)) toward Co(2+) with a linear range from 0.1 to 0.7 μM. The cost-effective method allows rapid and simple determination of the concentrations of Co(2+) ions in drinking water.