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 aminemodified DNA (NH 2 -oln). The absorbance trend with NH 2 -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 NH 2 -oln were sorted out with lower concentration (50 μM) of these amino acids. This observation was successfully transferred to differentiate 3 10 -helical Ac-(AAAAK) 3 A-NH 2 from α-helical Ac-(AAAAR) 3 A-NH 2 . The concept was further applied for the detection of single arginine modification closest to the carboxy terminus of 3 10 -helical Ac-(AAAAK) 3 A-NH 2 peptide for maximum conformational change toward α-helix.
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.
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. Graphical abstract 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.
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 Au 0 reduction, thus providing in-plane growth of seed. In contrast to this well-established growth mechanism, herein we report the unique case of methionine (Met) controlled seed mediated growth reaction, which rather proceeds via impeding secondary nucleation in presence of citrate stabilized gold nanoparticle (AuNP). The interaction between the freshly generated Au + and thioether group of Met in the medium restricts the secondary nucleation process of further seed catalyzed Au + reduction to Au 0 . This incomplete conversion of Au + , as confirmed by X-ray photoelectron spectroscopy (XPS), results in a significant enhancement of the zeta (z) potential even at low Met concentration. Nucleation of in situ generated small-sized particles (nAuNPs) takes place on the parent seed surface followed by their segregation from the seed. 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 towards the seed mediated growth reaction of AuNP in presence of three bio-inspired decameric peptides having varying number of 2 Met residues. The study confirms the nucleation strategy even in presence of single Met residue in the peptide and also the self-assembly of nucleated particles with increasing Met residues within the peptide.
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