Advanced wearable biosensors for the detection of body fluids and exhaled breath by graphene.

Given the huge economic burden caused by chronic and acute diseases on human beings, it is an urgent requirement of a cost-effective diagnosis and monitoring process to treat and cure the disease in their preliminary stage to avoid severe complications. Wearable biosensors have been developed by using numerous materials for non-invasive, wireless, and consistent human health monitoring. Graphene, a 2D nanomaterial, has received considerable attention for the development of wearable biosensors due to its outstanding physical, chemical, and structural properties. Moreover, the extremely flexible, foldable, and biocompatible nature of graphene provide a wide scope for developing wearable biosensor devices. Therefore, graphene and its derivatives could be trending materials to fabricate wearable biosensor devices for remote human health management in the near future. Various biofluids and exhaled breath contain many relevant biomarkers which can be exploited by wearable biosensors non-invasively to identify diseases. In this article, we have discussed various methodologies and strategies for synthesizing and pattering graphene. Furthermore, general sensing mechanism of biosensors, and graphene-based biosensing devices for tear, sweat, interstitial fluid (ISF), saliva, and exhaled breath have also been explored and discussed thoroughly. Finally, current challenges and future prospective of graphene-based wearable biosensors have been evaluated with conclusion. Graphene is a promising 2D material for the development of wearable sensors. Various biofluids (sweat, tears, saliva and ISF) and exhaled breath contains many relevant biomarkers which facilitate in identify diseases. Biosensor is made up of biological recognition element such as enzyme, antibody, nucleic acid, hormone, organelle, or complete cell and physical (transducer, amplifier), provide fast response without causing organ harm.

Methionine-Controlled Impediment of Secondary Nucleation Leading to Nonclassical Growth within Self-Assembled De Novo Gold Nanoparticles.

The conventional key steps for seed-mediated growth of noble metal nanostructures involve classical and nonclassical nucleation. Furthermore, the surface of the seed catalytically enhances the secondary nucleation involving Au+ to Au0 reduction, thus providing in-plane growth of the seed. In contrast to this well-established growth mechanism, herein, we report the unique case of a methionine (Met)-controlled seed-mediated growth reaction, which rather proceeds via impeding secondary nucleation in the presence of citrate-stabilized gold nanoparticles (AuNPs). The interaction between the freshly generated Au+ and the thioether group of Met in the medium restricts the secondary nucleation process of further seed-catalyzed Au+ reduction to Au0. This incomplete conversion of Au+, as confirmed by X-ray photoelectron spectroscopy, results in a significant enhancement of the zeta (ζ) potential even at low Met concentrations. Nucleation of in situ generated small-sized particles (nAuNPs) takes place on the parent seed surface followed by their segregation from the seed. The self-assembly process of these nAuNPs arises from the aurophilic interaction among the Au+. Furthermore, the time-dependent growth of smaller particles to larger-sized particles through assembly and merging within the same self-assembly validates the nonclassical growth. This strategy has been successfully extended toward the seed-mediated growth reaction of AuNPs in the presence of three bio-inspired decameric peptides having varying numbers of Met residues. The study confirms the nucleation strategy even in the presence of a single Met residue in the peptide and also the self-assembly of nucleated particles with increasing Met residues within the peptide.

Green Synthesis of Luminescent Gold-Zinc Oxide Nanocomposites: Cell Imaging and Visible Light-Induced Dye Degradation.

Green synthesis of gold-zinc oxide (Au-ZnO) nanocomposite was successfully attempted under organic solvent-free conditions at room temperature. Prolonged stirring of the reaction mixture introduced crystallinity in the ZnO phase of Au-ZnO nanocomposites. Luminescence properties were observed in these crystalline Au-ZnO nanocomposites due to in situ embedding of gold nanoparticles (AuNP) of 5-6 nm diameter on the surface. This efficient strategy involved the reduction of Au(III) by Zn(0) powder in aqueous medium, where sodium citrate (NaCt) was the stabilizing agent. Reaction time and variation of reagent concentrations were investigated to control the Au:Zn ratio within the nanocomposites. The reaction with the least amount of NaCt for a long duration resulted in Au-ZnO/Zn(OH)2 nanocomposite. X-ray photoelectron spectroscopy (XPS) confirmed the formation of Zn(OH)2 and ZnO in the same nanocomposite. These nanocomposites were reconnoitered as bioimaging materials in human cells and applied for visible light-induced photodegradation of rhodamine-B dye.

Tryptophan-Stabilized Au-FexOy Nanocomposites as Electrocatalysts for Oxygen Evolution Reaction.

Au-FexOy nanocomposites with a variable gold-to-iron ratio were stabilized with l-tryptophan. The synthetic methodology is based on the facile redox reaction between Au(III) and Fe(0) in the presence of gold nanoparticle as a seed at room temperature in an aqueous medium. The synthesis results in the deposition of Au nanoparticles on the surface of iron oxide layers. Composition variation in the nanocomposites was obtained by controlling the seed amount and reducing agent. These nanocomposites are used as electrocatalysts for the thermodynamically unfavorable oxygen evolution reaction (OER) from water. Among the nanocomposites, the most efficient OER activity was observed from the nanocomposite 12. The content of iron with respect to gold is at the maximum in the nanocomposite, which was obtained from the reaction with a minimum seed concentration and maximum reducing agent.

Gold Nanoflower for Selective Detection of Single Arginine Effect in α-Helix Conformational Change over Lysine in 310-Helix Peptide.

Hydroxylamine-based growth reaction in the presence of natural l-amino acids (9 mM) and gold nanoparticle seed mostly produce aggregated or nonaggregated gold nanostructures except the cases of immediate precipitation with aspartic acid, glutamic acid, cysteine, and tyrosine. Among the other amino acids, arginine shows the control growth reaction to form gold nanoflower from gold nanoparticle seeds, which were preincubated with amine-modified DNA (NH2-oln). The absorbance trend with NH2-oln in the presence of arginine is similar to the aggregation behavior in the presence of histidine and methionine. The formations of gold nanoflower with arginine and aggregation due to histidine and methionine in the presence of NH2-oln were sorted out with lower concentration (50 µM) of these amino acids. This observation was successfully transferred to differentiate 310-helical Ac-(AAAAK)3A-NH2 from α-helical Ac-(AAAAR)3A-NH2. The concept was further applied for the detection of single arginine modification closest to the carboxy terminus of 310-helical Ac-(AAAAK)3A-NH2 peptide for maximum conformational change toward α-helix.

Formation of Growth-Mediated Gold Nanoflowers: Roles of the Reducing Agent and Amine-Modified, Single-Strand DNA Sequences.

The formation of growth-mediated structures from gold nanoparticle seeds was studied in the presence of amine-modified single-strand DNA sequences and reducing agents such as hydroxylamine and hydroquinone. In the case of hydroxylamine, spherical gold nanoparticle seeds (0.45 nM) were incubated with amine-modified single-strand DNA probes PMR (amine-5'-ACATCAGT-3') and PML (amine-5'-GATAAGCT-3'), which resulted in gold nanoflowers and nanospheres, respectively. When the concentration of the nanoparticle seeds was varied (0.15-0.45 nM), only the PMR sequence showed growth-mediated development of gold nanoflowers. The size of the gold nanoparticles obtained is independent of the seed concentration for both PMR and PML sequences. In contrast, in the presence of the reducing agent hydroquinone, the growth processes are identical in for both the sequences. At a lower seed concentrations (0.15 nM), gold nanoflowers of larger size were observed for both sequences, whereas at higher seed concentrations (0.45 nM), much smaller gold nanospheres resulted. The formation and stability of nanoflowers and nanospheres for PMR and PML with hydroxylamine-based reduction were further studied in detail with diverse controlled amine-modified (5'-, 3'- and both end-modified) and non-modified DNA sequences with other mutants of these two sequences.