A novel fundamental understanding of the features of mechanism for the synthesis of luminescent silver nanoclusters (AgNCs) in human hemoglobin (Hb) as capping/reducing agents is presented based upon simultaneous size transition and fluorescence enhancement phenomena  The interesting features consist of both NC core oxidation and aggregation-induced emission (AIE) attributed to ligand-to-metal charge transfer (LMCT) or ligand-to-metal–metal charge transfer (LMMCT) from Ag­(I)-Hb complexes (through oxygen, nitrogen, and sulfur atoms of Hb residues donation to the Ag­(I) ions) forming Ag(0)@Ag­(I)–Hb core–shell NCs, the origin and consequence being a dual emission/single excitation nanosystem with large stocks shift and high quantum yield obtained even at high temperature which is a challenging subject,  is not reported until now. The bioconjugation of hyaluronic acid (HA) onto surfaces of an Hb layer (HA/AgNCs) produced a biocompatible platform with a doxorubicin drug (DOX) as DOX/HA/AgNCs for specific imaging and delivery of DOX via an efficient targeting of CD44-overexpressing cancer cells, which lead to an increased inhibition of tumor cell growth. Additionally, the cell viability analysis illustrated that the developed nanocarriers significantly enhanced the DOX uptake in HeLa cancer cells compared to HUVEC and HNCF-PI 52 normal cells allowing a selective cytotoxicity to HeLa cells. The suggested LMCT/LMMCT mechanism for an emission source combined with such attractive properties as a simple one-pot, nontoxic, synthesis route, long lifetime, large Stocks shift, excellent aqueous stability and photostability, and easy functionalization capability with good cell viability provided the possibility for a AgNCs nanoprobe for use to better understand the nucleation and growth mechanisms via computational modeling techniques (e.g., DFT study) and also for fabrication of new nanoprobes for developing multifunctional applications in the biobased chemical and electrochemical fields and in in vivo research.

Four fluorescent DNA-stabilized fluorescent silver nanoclusters (DNA–AgNCs) were designed and synthesized with differences in lengths of cytosine-rich DNA strand (as the stabilizing agent) and target-specific strand DNA aptamers for adenosine triphosphate (ATP) and cytochrome c (Cyt c). After their nanohybrid formation with graphene oxide (GO), it was unexpectedly found that, depending on the composition of the base and length of the strand DNA aptamer, the fluorescence intensity of three of the nanohybrids significantly enhanced. Our experimental observations and quantum mechanical calculations provided an insight into the mechanisms underlying the behavior of DNA–AgNCs/GO nanohybrids. The enhanced fluorescence was found to be attributed to the aggregation-induced emission enhancement (AIE) characteristic of the DNA–AgNCs adsorbed on the GO surface, as confirmed evidently by both fluorescence and transmission electron microscopies. The AIE is a result of hardness and oxidation properties of GO, which lead to enhanced argenophilic interaction and thus to increased Ag­(I)–DNA complex shell aggregation. Consequently, two of the DNA–AgNCs/GO nanohybrids were successfully extended to construct highly selective, sensitive, label-free, and simple aptasensors for biosensing of ATP (LOD = 0.42 nM) and Cyt c (LOD = 2.3 nM) in lysed Escherichia coli DH5 α cells and mouse embryonic stem cells, respectively. These fundamental findings are expected to significantly influence the designing and engineering of new AgNCs/GO-based AIE biosensors.

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