Enzyme-linked immunosorbent assay (ELISA) is a technique designed for the detection and quantification of (bio)­molecules in a liquid sample. It is a powerful tool in clinical diagnostics, food safety, and environmental monitoring. However, the main limitation of conventional ELISA is its low sensitivity, which cannot meet the demand of analyte analysis in complex (biological) samples. Several successful approaches capitalizing on the unique physical and chemical properties of nanoparticles to improve the performance of traditional ELISA have been reported. In this review, we aim to demonstrate diverse strategies designed to date that use metal and metal oxide nanoparticles to overcome challenges associated with ELISA sensitivity and stability. In particular, we discuss metal and metal oxide nanoparticles as carriers to load enzymes and antibodies for signal amplification, as enzyme mimics to replace the natural enzyme label, and as signal transducers to provide fluorescence or luminescence signals as an alternative output.

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Sensitivity is the key in optical detection of low-abundant analytes, such as circulating RNA or DNA. The enzyme Exonuclease III (Exo III) is a useful tool in this regard; its ability to recycle target DNA molecules results in markedly improved detection sensitivity. Lower limits of detection may be further achieved if the detection background of autofluorescence can be removed. Here we report an ultrasensitive and specific method to quantify trace amounts of DNA analytes in a wash-free suspension assay. In the presence of target DNA, the Exo III recycles the target DNA by selectively digesting the dye-tagged sequence-matched probe DNA strand only, so that the amount of free dye removed from the probe DNA is proportional to the number of target DNAs. Remaining intact probe DNAs are then bound onto upconversion nanoparticles (energy donor), which allows for upconversion luminescence resonance energy transfer (LRET) that can be used to quantify the difference between the free dye and tagged dye (energy acceptor). This scheme simply avoids both autofluorescence under infrared excitation and many tedious washing steps, as the free dye molecules are physically located away from the nanoparticle surface, and as such they remain "dark" in suspension. Compared to alternative approaches requiring enzyme-assisted amplification on the nanoparticle surface, introduction of probe DNAs onto nanoparticles only after DNA hybridization and signal amplification steps effectively avoids steric hindrance. Via this approach, we have achieved a detection limit of 15 pM in LRET assays of human immunodeficiency viral DNA.

The detection of pathogenic bacteria is essential to prevent and treat infections and to provide food security. Current gold-standard detection techniques, such as culture-based assays and polymerase chain reaction, are time-consuming and require centralized laboratories. Therefore, efforts have focused on developing point-of-care devices that are fast, cheap, portable and do not require specialized training. Paper-based analytical devices meet these criteria and are particularly suitable to deployment in low-resource settings. In this Review, we highlight paper-based analytical devices with substantial point-of-care applicability for bacteria detection and discuss challenges and opportunities for future development.

We report the synthesis of a catalyst, copper-doped zeolitic imidazolate framework ZIF-8, that generates nitric oxide from naturally occurring endogenous nitric oxide donors, S-nitrosoglutathione and S-nitrosocysteine.

half-life (<5 s) and diffusion distance (40-200 µm) in vitro and in vivo. [7] In addition, the biological functions of NO are pleiotropic and strongly dependent on its concentrations. At lower concentrations (pm to nm), NO promotes cell survival and proliferation, [8,9] while higher NO concentrations (µm to mm) cause cell apoptosis [10] with potential for anticancer, [11] antibacterial, [12] and antiviral applications. [13] To this end, researchers have developed a number of NO-releasing platforms where NO donors, denoted as substances that transport and release NO upon specific stimulus, are encapsulated into pre-fabricated scaffolds (e.g., silica nanoparticles, polymers, liposomes, hydrogels, and metal-organic frameworks). [8,[14][15][16][17] Even though these platforms have been demonstrated to enable controlled and sustained NO release profiles, [18][19][20] their NO payloads and duration of NO release principally rely on the finite amount of NO donors incorporated. To tackle this issue, NO delivery by catalytic approaches have been developed, which enables in situ continuous NO generation from naturally occurring endogenous NO donors S-nitrosothiols (RSNOs) mediated by catalytic reagents. [21] One of the well-known catalytic approaches for NO generation is the copper-facilitated decomposition of S-nitrosothiols, where transition Cu 2+ is reduced to Cu + by thiolate, and Cu + subsequently reacts with RSNO to release NO and regenerate Cu 2+ . [22] This discovery has led to the development of copper-based materials for NO release from S-nitrosoglutathione (GSNO), [23,24] S-nitrosocysteamine, [25,26] S-nitrosocysteine (CysNO), [27] and S-nitroso-Nacetyl-DL-penicillamine (SNAP). [28] Another representative catalytic agent for NO release is gold nanoparticles. Catalytic gold nanoparticles were reported to catalyze NO release from GSNO, SNAP, and S-nitrosopenicillamine, which was ascribed to the formation of Au-thiolate and the oxidation of Au 0 to Au [1] on the surface of gold nanoparticles. [29,30] Recently, Doverspike et al. [31] reported that zinc oxide particles enhanced NO release from GSNO, and our group [32,33] presented the ability of zinc oxide particles to catalytically decompose both endogenous (GSNO) and exogenous (β-gal-NONOate) donors to generate NO at physiological conditions. Intriguingly, the opposite capability, that is, NO-scavenging capability, of gold nanoparticles, [34] copper nanoparticles, [35] and zinc oxide particles [36,37] in the presence of excessive NO were also reported, which shows the multi-faceted functions of these transition metal nanoparticles.Ceria nanoparticles (NPs) are widely reported to scavenge nitric oxide (NO) radicals. This study reveals evidence that an opposite effect of ceria NPs exists, that is, to induce NO generation. Herein, S-nitrosoglutathione (GSNO), one of the most biologically abundant NO donors, is catalytically decomposed by ceria NPs to produce NO. Ceria NPs maintain a high NO release recovery rate and retain their crystalline structure for at least 4 w...

Hydrogen sulfide (H2S) is a gaseous molecule involved in multiple biological and physiological processes, including various diseases such as cancer and neurodegenerative disorders. This has led to the development of various analytical methods to monitor H2S in biological settings. Among these, fluorescence‐based assays, specifically organic small‐molecule probes, have been thoroughly utilized. They offer good sensitivity and specificity as sensors, and noninvasive detection with high spatiotemporal resolution in in vitro and in vivo imaging. Despite attempts to decrease the rate of photobleaching and enhance the photostability of these dyes, they are still limited by low survival time and complex reagent pretreatment. Fortunately, nanotechnology has been applied to develop effective, highly sensitive, and specific fluorescent nanoprobes. Specifically, nanomaterial‐based H2S probes have emerged as promising candidates for real‐time detection and imaging. In contrast to their organic molecule‐based counterparts, they offer higher versatility in imaging modes due to their unique optical properties, improved photostability and solubility within physiological fluids, as well as easily modifiable surfaces and tuneable structures for improved specificity. Recently, many nanomaterial‐based probes, ranging from inorganic nanoparticles to self‐assembled nanocomposites, have been developed. These have, for the most part, achieved sensitive and specific endogenous H2S detection and in vivo imaging. In this review, we evaluate five different nanomaterials currently being researched to detect and image endogenous H2S within the last 5 years. Furthermore, analytical methods associated with the various signal outputs, current challenges in H2S nanoprobe design, and possible future research interests are outlined and discussed.

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