This article reviews the principles, design and applications of visible-light and near-infrared fluorescence and surface-enhanced Raman scattering in point-of-care testing and bio-imaging. 

Visible-light and near-infrared fluorescence and surface-enhanced Raman scattering point-of-care sensing and bio-imaging: a review

Rapid, efficient, and accurate detection of foodborne bacteria is important for public health and food safety; however, point-of-care testing remains a limiting factor in the field. Compared with traditional methods, the use of microfluidic chips in monitoring foodborne bacteria has the advantages of miniaturization, automation, integration, high throughput, and low consumption. Notably, the combination of nanomaterials with microfluidics further provides an improved method of pathogen detection with superior, rapid, and sensitive detection efficiencies. This review presents a broad overview of a wide range of microfluidic chips, including the magnetic nanomaterial-based assay, metal nanomaterial-based assay, carbon nanomaterial-based assay, quantum dot-based assay, and upconversion nanoparticle-based assay, for the detection of foodborne bacteria using different nanomaterials. Additionally, the main challenges that exist for translation of scientific research into product development are discussed. On this basis, we provide perspectives on both future technological directions and potential applications, which will guide researchers in identifying the most promising areas of development in this field. 

Advances in nanomaterial-based microfluidic platforms for on-site detection of foodborne bacteria

An intriguing self-powered cathodic photoelectrochemical (PEC) microfluidic aptasensor with enhanced cathodic photocurrent response is proposed for sensitive detection of prostate-specific antigen (PSA). The self-powered system is constructed by a cadmium sulfide-sensitized zinc oxide nanorod array (CdS/ZnO NA) as a photoanode with an iodide-doped bismuth oxychloride flower-array (I0.2:BiOCI0.8) as a photocathode, which can generate the electrical output under visible light irradiation with no external power supply. In addition, the p-type semiconductor I0.2:BiOCI0.8 with a special internal electric field between the iodide ion layer and the [Bi2O2]2+ layer could increase the cathodic photocurrent response by facilitating the separation of electron/hole pairs under visible light excitation. It is worth noting that dissolved oxygen as an electron acceptor can be reduced by the photogenerated electron to form a superoxide radical (•O2-) in the self-powered cathodic PEC system. The further enhanced cathodic photocurrent response can be achieved by eliminating •O2- that reacts with the luminol anion radical (L•-) to produce chemiluminescence emission, which serves as an inner excitation light source. What is more exciting is that the integration of the photoanode and the photocathode into a microfluidic chip could realize automatic sample injection and detection. On this basis, the proposed aptasensor presents excellent reproducibility and high sensitivity for detecting PSA and exhibits a good linearity range (50 fg·mL-1 to 50 ng·mL-1) with a low detection limit (25.8 fg·mL-1), which opens up a new horizon of potential for sensitively detecting other kinds of disease markers.

Self-Powered Cathodic Photoelectrochemical Aptasensor Comprising a Photocathode and a Photoanode in Microfluidic Analysis Systems.

Infectious virus outbreaks pose a significant challenge to public healthcare systems. Early and accurate virus diagnosis is critical to prevent the spread of the virus, especially when no specific vaccine or effective medicine is available. In clinics, the most commonly used viral detection methods are molecular techniques that involve the measurement of nucleic acids or proteins biomarkers. However, most clinic‐based methods require complex infrastructure and expensive equipment, which are not suitable for low‐resource settings. Over the past years, smartphone‐based point‐of‐care testing (POCT) has rapidly emerged as a potential alternative to laboratory‐based clinical diagnosis. This review summarizes the latest development of virus detection. First, laboratory‐based and POCT‐based viral diagnostic techniques are compared, both of which rely on immunosensing and nucleic acid detection. Then, various smartphone‐based POCT diagnostic techniques, including optical biosensors, electrochemical biosensors, and other types of biosensors are discussed. Moreover, this review covers the development of smartphone‐based POCT diagnostics for various viruses including COVID‐19, Ebola, influenza, Zika, HIV, et al. Finally, the prospects and challenges of smartphone‐based POCT diagnostics are discussed. It is believed that this review will aid researchers better understand the current challenges and prospects for achieving the ultimate goal of containing disease‐causing viruses worldwide.

Virus Detection: From State‐of‐the‐Art Laboratories to Smartphone‐Based Point‐of‐Care Testing

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

Recent Advances in Microfluidic Technology for Bioanalysis and Diagnostics.

Abstract Gold nanoparticles (AuNPs) are employed in numerous applications, including optics, biosensing and catalysis. Here, we demonstrate the stabilizer‐free electrochemical synthesis of AuNPs inside nanoliter‐sized reactors. Droplets encapsulating a gold precursor are formed on a microfluidic device and exposed to an electrical current by guiding them through a pair of electrodes. We exploit the naturally occurring recirculation flows inside confined droplets (moving in rectangular microchannels) to prevent the aggregation of nanoparticles after nucleation. Therefore, AuNPs with sizes in the range of 30 to 100 nm were produced without the need of additional capping agents. The average particle size is defined by the precursor concentration and droplet velocity, while the charge dose given by the electric field strength has a minor effect. This method opens the way to fine‐tune the electrochemical production of gold nanoparticles, and we believe it is a versatile method for the formation of other metal nanoparticles.

Maximilian Breitfeld

Papers

Recent Advances in Microfluidic Technology for Bioanalysis and Diagnostics.

Continuous Electroformation of Gold Nanoparticles in Nanoliter Droplet Reactors