Computational modelling of gold complexes using density functional theory

Synthesis and characterization of gold(I) thiolate derivatives and bimetallic complexes for HIV inhibition

Introduction: The human immunodeficiency virus (HIV) remains a significant global health concern, with a reported high infection rate of 38.4 million cases globally; an estimated 2 million new infections and approximately 700,000 HIV/AIDS-related deaths were reported in 2021. Despite the advent of anti-retroviral therapy (ART), HIV/AIDS persists as a chronic disease. To combat this, several studies focus on developing inhibitors targeting various stages of the HIV infection cycle, including HIV-1 protease. This study aims to synthesize and characterize novel glyco diphenylphosphino metal complexes with potential HIV inhibitory properties. Method: A series of new gold(I) thiolate derivatives and three bimetallic complexes, incorporating amino phosphines and thiocarbohydrate as auxiliary ligands, were synthesized using procedures described by Jiang, et al. (2009) and Coetzee et al. (2007). Structural elucidation and purity assessment of the synthesized compounds (1-11) were conducted using micro-analysis, NMR, and infrared spectrometry. Results and Discussion: Using molecular modeling techniques, three of the metal complexes were identified as potential HIV protease inhibitors, exhibiting strong binding affinity interactions with binding pocket residues. These inhibitors demonstrated an ability to inhibit the flexibility of the flap regions of the HIV protease, similar to the known HIV protease inhibitor, darunavir. This study sheds light on the promising avenues for the development of novel therapeutic agents against HIV/AIDS.

Upconversion luminescence nanosensor for detection of Fe3+ and phosphate ion based on the inner-filter effect

In this work, an upconversion luminescence (UCL) nanosensor for fast detection of ferric ion (Fe3+) and phosphate ion (Pi) is developed based on the inner-filter effect (IFE) between NaYF4:Yb/Er upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex. Fe3+-HB complex has strong absorption band (450-650 nm), which overlaps with the green emission peak of UCNPs at 545 nm. By adding Fe3+ and Pi, the UCNPs-HB system produces the red-shift change of absorption spectrum, which leads to the "on-off-on" process of IFE. So, with the specific recognition ability of HB for Fe3+ and the competitive complexation of Pi for Fe3+, the proposed nanosensor utilizes the UCL change to achieve the detection of the targets. For the detections of Fe3+, the linear range is 10-600 μM with a limit of detection (LOD) of 2.62 μM, and for Pi, the linear range is 5-100 μM with a LOD of 1.25 μM. The results for selectivity, precision, and recovery test are also satisfactory. Furthermore, the real sample detection shows that the proposed nanaosensor has a great potential in environmental and biological systems. An upconversion luminescence (UCL) nanosensor based on the inner-filter effect (IFE) between upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex for the detection of Fe3+ and phosphate ion has been proposed, which is promising to be a convenient and sensitive assay for monitoring Fe3+ and phosphate ion in different environments and biological systems.

Size-modulated optical property of gold nanorods for sensitive and colorimetric detection of thiourea in fruit juice

As an important sulfur compound, thiourea (TU) has caused great concern because of its wide application as well as its serious toxicity and hazard to the environment. Thus, it is necessary to develop a sensitive and selective method for TU analysis. In this work, gold nanorods (AuNRs) acted as an optical probe to realize the sensitive and colorimetric detection of TU. In HCl medium, Fe³⁺ at low concentration was difficult to oxide Au⁰ to form Au⁺ because of the high redox potential or the positive Gibbs free energy change. However, this process was possible when TU was present since the association constant between Au⁺ and TU is great enough to bind with TU to form a stable complex to further promote the etching of AuNRs, resulting in the lower aspect ratio of AuNRs with the blue shift and intensity decrease in extinction spectra, accompanied by the divisive colors of AuNRs solution or colorful dark-field light scattering imaging of single AuNR. The blue-shift of AuNRs longitudinal plasmon resonance absorption (LPRA) band was proportional to the concentration of TU in the range of 1-250 nM and the limit of detection (3σ/k) was as low as 0.4 nM. In addition, the colorimetric method was proven with high selectivity in the presence of potential interfering compounds, which was successfully applied to the detection of TU in fruit juice samples. This proposed colorimetric method provides a simple, sensitive yet selective measurement tool for TU sensing, which may offer new opportunities in the development of colorimetric sensors for food safety in the future.

Increased Recovery of Gold Thiosulfate Alkaline Solutions by Adding Thiol Groups in the Porous Structure of Activated Carbon

Thiosulfate leaching combined with ion-exchange resins is an innovative alternative for gold recovery. According to the properties of activated carbon, it could replace resins in the gold recovery process, improve efficiency, and reduce operating cost. In this research, the adsorption process of gold thiosulfate complex on thiol-modified activated carbon was studied. Thioglycolic acid (ATG) was impregnated in activated carbon, and its adsorption ability was tested with synthetic solutions of gold and sodium thiosulfate (Au 10 mg·L^-1, Na₂S₂O₃ 0.1 mol·L^-1, pH = 10.0). Carbon was characterized by infrared spectroscopy, SEM-EDS, PZC titration, hardness number measures, and proximal analysis. Synthetic solutions were also characterized by UV-vis spectroscopy and cyclic voltammetry. The percentage of volatile material increased from 10.0 to 13.9% due to the impregnation process of ATG. Infrared spectra show characteristic bands of C-H, S-H, and C-S bonds. In the adsorption tests, the ATG-impregnated carbon achieved 91% of gold recovery, while the same amount of ATG in the liquid phase stirred with unmodified activated carbon reached 90% of gold recovery. The 44.9% of gold recovered with activated carbon impregnated with ATG was eluted with sodium cyanide ([NaCN] = 0.2 mol·L^-1; [NaOH] = 0.25 mol·L^-1; [CH₃CH₂OH] = 30% V/V; pH = 12.0; t = 24 h). These results suggest the gold transferred from the thiosulfate complex to a new gold thiolate complex.

[Au(CN)2]—Adsorption on a Graphite (0001) Surface: A First Principles Study

Gold is mainly present in the form of [Au(CN)2]− during the cyanide leaching process, and this [Au(CN)2]− can be adsorbed by graphite in carbonaceous gold ore resulting in preg-robbing gold. In order to clarify the adsorption mechanism between the [Au(CN)2]− and graphite, the interaction between the [Au(CN)2]− and graphite (0001) surface was studied using density functional theory (DFT). The distance between [Au(CN)2]− and graphite (0001) decreased from (4.298–4.440 Å) to (3.123–3.343 Å) after optimization, and the shape of [Au(CN)2]− and graphite (0001) obviously changed from straight to curved, which indicated that the [Au(CN)2]− had been adsorbed on the graphite (0001) surface. A partial densities of state (PDOS) analysis revealed that there was little change in the delocalization and locality of the PDOS on the graphite (0001) surface after adsorption. However, the valence bands of the Au 5d orbital, C 2p orbital, and N 2p orbital near the Fermi level moved slightly towards lower energy levels; therefore, the adsorption configuration was stable. An analysis of the Mulliken charge population indicated that the Au, N, and C in [Au(CN)2]− obtained 0.26, 0.18, 0.04 electrons after adsorption, respectively, while C(surf) lost 0.03 electrons. [Au(CN)2]− changed to a conductor from an insulator after adsorption. Taking into account the surface electrical properties of [Au(CN)2]− and graphite (0001), there was still a slight electrostatic adsorption between them. The analysis of adsorption energy, electronic structure, PDOS, electron density, Mulliken charge population, and Mulliken bond population revealed that [Au(CN)2]− could be adsorbed to the graphite (0001) surface; the adsorption was a type of physical adsorption (including electrostatic adsorption) and mainly occurred on the two C≡N. These results contributed to the understanding of the mechanisms involved in preg-robbing gold formation by graphite and the optimization of this process during cyanide leaching.

Real-time dark-field light scattering imaging to monitor the coupling reaction with gold nanorods as an optical probe

Gold nanorods (GNRs) have opened up promising applications based on their reshaping, due to the fact that a tiny change in shape or size could directly lead to optical changes. Herein, we report chemical reshaping of GNRs induced by the coupling reaction between Au, ferric chloride and thiourea. In the coupling reaction, Fe³⁺ oxidizes the GNRs to yield Au(i), which complexes with the thiourea ligand, lowering the Gibbs free energy of the gold species and promoting the reaction equilibrium to enable the chemical reshaping of the GNRs. This coupling reaction process was monitored using a light-scattering dark-field microscopy (DFM) imaging technique and scanning electron microscopy (SEM). The light scattering underwent a colour change from bright red to yellow and finally to green, and the GNRs underwent a morphological change from rod-shaped to fusiform and finally to spherical, which is somewhat different from the results of other chemical etching processes of GNRs. It is believed that the coupling reaction induced chemical reshaping of GNRs not only provides an alternative way to monitor the coupling reaction, but also offers a facile way to obtain a desirable GNR morphology, which is important for the preparation of fusiform nanostructures.

Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4](-): Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4](-) reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4](-) and (2) radical-mediated reduction of [AuCl4](-) upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4](-) under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4](-) at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.