In vitro diagnostics (IVD) that performs a diagnostic test outside of a living body plays a crucial role in clinical care. Although the past few years have witnessed a rapidly growing requirement in the development of IVD tools with high analytical figures of merit, conventional IVD platforms are still facing several limitations, such as time-consuming procedures, the requirement of expensive reagents, moderate sensitivity, and sophisticated instruments. The advancement of nanotechnology has led to tremendous progress in the design of cutting-edge IVD platforms, which show distinct features including unmatched sensitivity, high multiplex capability, easy to use and low cost.
Fluorescence-based readout assays play an essential role in life sciences and medicine because of their versatility and practical applicability. Despite their ubiquity, continuous efforts are needed to improve both the sensitivity and specificity. Among them, nanostructure-based fluorescence enhancement platforms have been developed for this purpose. The nanomaterials so far predominantly explored are primarily metallic nanoparticles, providing plasmonic-induced fluorescent enhancement. In the context of optical biosensor applications, semiconductor nanowires have recently been paid increased attention due to their highly advantageous optical properties. Specifically, nanowires have been observed to collect the fluorescence emission of a large number of surface-bound fluorophores and re-emit it at their tip. This process greatly enhances the overall intensity of the emission and even enabling single-molecule detection without advanced optics. Several studies employing different semiconductor materials, such as ZnO, Si, GaP, InAs and GaAs, have addressed the potential of nanowires to enhance fluorescent signals.
In vitro biosensors have been an important component for the early diagnosis of cancer in the clinic. Among them, no-wash biosensor assays are conducted through simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, which makes them particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point of care testing (POCT). Nanotechnology has improved the specificity, sensitivity and multiplexing detection capacity of no-wash biosensors. The usage of nanomaterials as signal amplification carriers or direct signal generating elements is one fast-growing field of no-wash biosensors.
Fluorescent metal nanoclusters (MNCs) are an emerging class of luminescent nanomaterials that are composed of several to a few hundred atoms. Recently, MNCs have attracted great interest due to their special physical and chemical properties, optical and electronical properties, and molecule-like characteristics. These excellent properties make them ideal fluorescent nanomaterials for promising applications in biological analysis
Due to their ultrasmall size, bright photoluminescence, good biocompatibility, and photostability, gold nanoclusters (AuNCs) have drawn increasing attention as a generation of fluorescent probes for biolabeling and biosensing. AuNC-based fluorescence biosensors have also been used to test other tumor-related enzymes.
Silver nanoclusters (AgNCs) are brighter and can be easily synthesized with different ligands. AgNC-based fluorescent no-wash biosensors mainly involve the following four signaling methods: nanocluster molecular beacon-based biosensors, DNA-templated AgNC in situ synthesis-based biosensors, hybridization enhanced fluorescence-based biosensors using guanine-rich DNA.
Copper nanoclusters (CuNCs) have also been extensively used as promising fluorescence probes for detecting various targets, such as proteins, single nucleotide polymorphisms (SNPs), miRNAs, cancer cells, and so on.
Fluorescence quenching-based no-wash biosensors are one of the most important energy-transfer-based-analytical platforms, where the fluorescence of the donor can be effectively quenched by the acceptor in the presence or absence of target analytes. Recent reports have demonstrated the efficiency of fluorescence quenching of the fluorescence of various fluorescence molecules has been increased in the presence of various nanomaterial quenchers, including AuNPs, carbon nanomaterials (such as carbon nanotubes), graphene oxide (GO), carbon nanoparticles (CNPs) and other nanoquenchers.
In recent years, an increasing number of nanomaterials, including graphene-like 2D layered nanomaterials and metal-organic frameworks (MOFs), have attracted great interest as fluorescence quenchers in fluorescence quenching sensors. Graphene-like 2D layered nanomaterials, such as graphite-carbon nitride (g-C3N4) nanosheets and various layered transition metal dichalcognides (TMDs), have been studied for fluorescence quenching-based biosensors because of their similar 2D structures and excellent optical properties.
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