Precision Nanocluster-Based Toroidal and Supertoroidal Frameworks Using Photocycloaddition-Assisted Dynamic Covalent Chemistry.

Atomically precise nanoclusters (NCs) have recently emerged as ideal building blocks for constructing self-assembled multifunctional superstructures. The existing structures are based on various non-covalent interactions of the ligands on the NC surface, resulting in inter-NC interactions. Despite recent demonstrations on light-induced reversible self-assembly, long-range reversible self-assembly based on dynamic covalent chemistry on the NC surface has yet to be investigated. Here, it is shown that Au25 NCs containing thiolated umbelliferone (7-hydroxycoumarin) ligands allow [2+2] photocycloaddition reaction-induced self-assembly into colloidal-level toroids. The toroids upon further irradiation undergo inter-toroidal reaction resulting in macroscopic supertoroidal honey-comb frameworks. Systematic investigation using electron microscopy, atomic force microscopy (AFM), and electron tomography (ET) suggest that the NCs initially form spherical aggregates. The spherical structures further undergo fusion resulting in toroid formation. Finally, the toroids fuse into macroscopic honeycomb frameworks. As a proof-of-concept, a cross-photocycloaddition reaction between coumarin-tethered NCs and an anticancer drug (5-fluorouracil) is demonstrated as a model photo-controlled drug release system. The model system allows systematic loading and unloading of the drug during the assembly and disassembly under two different wavelengths. The results suggest that the dynamic covalent chemistry on the NC surface offers a facile route for hierarchical multifunctional frameworks and photocontrolled drug release.

Gold atomic cluster mediated electrochemical aptasensor for the detection of lipopolysaccharide.

We have constructed an aptamer immobilized gold atomic cluster mediated, ultrasensitive electrochemical biosensor (Apt/AuAC/Au) for LPS detection without any additional signal amplification strategy. The aptamer self-assemble onto the gold atomic clusters makes Apt/AuAC/Au an excellent platform for the LPS detection. Differential pulse voltammetry and EIS were used for the quantitative LPS detection. The Apt/AuAC/Au sensor offers an ultrasensitive and selective detection of LPS down to 7.94 × 10-21M level with a wide dynamic range from 0.01 attomolar to 1pM. The sensor exhibited excellent selectivity and stability. The real sample analysis was performed by spiking the diluted insulin sample with various concentration of LPS and obtained recovery within 2% error value. The sensor is found to be more sensitive than most of the literature reports. The simple and easy way of construction of this sensor provides an efficient and promising detection of an even trace amount of LPS.

Electrochemical synthesis of a gold atomic cluster-chitosan nanocomposite film modified gold electrode for ultra-trace determination of mercury.

Gold atomic cluster based nanocomposites are important in the field of energy and sensing applications due to their interesting optical, electronic, chemical and catalytic properties. In the present study a chitosan stabilized gold atomic cluster nanocomposite was synthesised by a simple electrochemical technique based on the anodic dissolution of a gold electrode in the presence of a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), and a biopolymer, chitosan, on a gold electrode. The gold clusters formed were characterized by DLS, TEM, MALDI-TOF-MS, XPS, fluorescence and cyclic voltammetry. The developed gold atomic cluster-chitosan (AuAC-Chit) nanocomposite modified gold electrode was highly sensitive and selective for the electrochemical detection of Hg(ii) ions. It offers a wider calibration range of 10(-14)-10(-7) M with a limit of detection (LOD) of 0.8 × 10(-14) M and a limit of quantification (LOQ) of 6.6 × 10(-14) M, much below the guideline value of 1 × 10(-8) M stipulated by United States Environmental Protection Agency (USEPA), accompanied by a good precision of 1.06% for 10(-13) M of Hg(ii). The designed sensor is selective to Hg(ii) ions in the presence of other coexisting species.

Ultrasensitive voltammetric determination of catechol at a gold atomic cluster/poly(3,4-ethylenedioxythiophene) nanocomposite electrode.

A novel gold atomic cluster-poly(3,4-ethylenedioxythiophene) (AuAC/PEDOT/Au) nanocomposite modified gold electrode has been designed for the trace level sensing of catechol. The addition of copper(II) enhanced the electro-catalytic oxidation of catechol via the formation of copper(I). The electrochemically synthesized PEDOT/Au and the AuAC/PEDOT/Au hybrid films were characterized by electrochemical and morphological methods. Under optimal conditions the nanocomposite modified electrode offers a wider calibration range of 1 × 10(-4) to 10 µM with a lowest detection limit of 6.3 pM for catechol. Moreover, the developed electrochemical sensor exhibited good selectivity and acceptable reproducibility (1.23% for 1 nM of catechol) and could be used for the routine detection and quantification of catechol in natural water samples. To gain a better understanding of such an excellent sensor performance achieved with this electrode, studies were undertaken to pinpoint electrode kinetics of charge transfer processes.

Mixed monolayer protected gold atom-oxide cluster synthesis and characterization.

Small atomic gold clusters in solution, Au(n), stabilized by cetyl trimethylammonium bromide (CTAB) and cysteine, have been synthesized potentiodynamically in quiescent aqueous solutions. The electrodissolution of gold to gold ions during an anodic scan and subsequent cluster formation during a cathodic scan in underpotential (UPDD) and overpotential dissolution-deposition (OPDD) regions were studied. The experimental potentiodynamic I-E profiles and chronoamperometric i-t transients are fit into reported theoretical models of adsorption and electrocrystallization. The plausible application of clusters/cluster film to cysteine sensing based on fluorescence quenching and square wave stripping voltammetry is demonstrated.

Hybrid gold atomic cluster-cobalt oxide scaffolds for dual tandem electrocatalytic sensing of cysteine.

Innovations in nanoelectrochemistry have opened a plethora of techniques for the preparation of nanometer sized electrochemical probes with tunable properties. Atomic clusters emerged at the crossroad of physics and chemistry has immense potential as novel "electron antennaes" in electrocatalysis. Herein, we report the electrochemical synthesis and the synergistic electrocatalytic effect of metal oxide supported gold atomic clusters for subnanomolar level sensing of cysteine. The developed gold atomic clusters and the hybrid materials were characterized by spectral, morphological and electrochemical methods. Comparative evaluation of the critical role of surfactants in stabilizing the hybrid material and the mechanistic aspects of support scaffolds has been discussed. Different complexing agents were sequentially screened in order to get the optimum electrolyte composition for developing a uniform, reproducible sensing film. The developed modified electrode exhibits highly reproducible wider calibration range of 10(-10) to 10(-6) M with lower detection limits of 1.6×10(-11) M for cysteine. To gain a greater understanding of such advantageous behavior achieved with this electrode, studies on electrode kinetics of charge-transfer processes were also done.