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  1. Abstract

    Gold nanoparticles (AuNPs) synthesized in the 1–3 nm range have a specific number of gold core atoms and outer protecting ligands. They have become one of the “hot topics” in recent decades because of their interesting physical and chemical properties. The characterization of their structures is usually achieved by crystal X‐ray diffraction although the structures of some AuNPs remain unknown because they have not been successfully crystallized. An alternative method for studying the structure of AuNPs is electrospray ionization–ion mobility–tandem mass spectrometry (ESI‐IM‐MSMS). This research evaluated how effectively ESI‐IM‐MSMS using the commercially available Waters Synapt XS instrument yielded useful structural information from two AuNPs; Au23(S‐tBu)16and Au30(S‐tBu)18. The study used the maximum range of available collision energies along with ion mobility separation to measure the energy‐dependence of the product ions and their drift times which is a measure of their spatial size. For Au23(S‐tBu)16, the dissociation gave the masses of the outer protecting monomeric [RS–Au–SR] and trimeric [SR–Au–SR–Au–SR–Au–SR] staples where R = tBu, and complete dissociation of the outer layer Au andtBu groups to reveal the Au15S8core. For Au30(S‐tBu)18, the dissociation products was primarily through the loss of the partial ligands S‐tBu andtBu from the outer protecting layer and the loss of single Au4(S‐tBu)4unit. These results showed the that ESI‐IM‐MSMS analysis of the smaller Au23(S‐tBu)16gave information on all it major structural components whereas for Au30(S‐tBu)18, the overall structural information was limited to the ligands of the outer layer.

     
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    Free, publicly-accessible full text available February 1, 2025
  2. Atomically precise thiolate-protected gold nanomolecules have attracted interest due to their distinct electronic and chemical properties. The structure of these nanomolecules is important for understanding their peculiar properties. Here, we report the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert -butylthiolate ligands. The composition of Au 24 (S-C 4 H 9 ) 16 {poly[hexadecakis(μ- tert -butylthiolato)tetracosagold]} was confirmed by X-ray crystallography and electrospray ionization mass spectrometry (ESI–MS). The nanomolecule was synthesized in a one-phase synthesis and crystallized from a hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core protected by two monomeric and two trimeric staples with four bridging ligands. The Au 24 (S-C 4 H 9 ) 16 cluster follows the shell-closing magic number of 8. 
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  3. Digestive ripening (DR) is a synthetic method where a polydisperse colloid of metal nanoparticles upon refluxing with a free ligand in a high boiling point solvent gives monodisperse nanoparticles. Brust synthesis is known to form atomically monodisperse thiolate protected gold nanoparticles also known as gold nanomolecules (Au NMs). Unlike the Brust method which gives smaller (1–3 nm) atomically precise nanomolecules, DR has been used only for the synthesis of large nanoparticles (>5 nm) with good monodispersity. In thiolate protected gold nanoparticle Brust synthesis, the yellow colored phase transferred Au( iii ) solution is converted to a colorless Au( i ) mixture after the addition of thiol by forming Au–SR, which is then reduced to form black colored Au NMs. However, in DR, by using the same primary chemicals, the two steps were reversed: the mixture was reduced before the addition of thiol. Here we show that in DR, adding thiol after 2 minutes of reduction gives larger particles (5 nm) as reported, whereas adding thiol 30 seconds after reduction results in smaller particles (<2 nm). In this work, for the first time, DR yields atomically precise Au 25 (SR) 18 and Au 144 (SR) 60 NMs. This is reported using two aliphatic thiols – hexanethiol and dodecanethiol – as the protecting ligands. DR was also repeated using an aromatic thiol, 4- tert -butyl benzene thiol (TBBT), which yields Au 279 (SR) 84 NMs consistent with the Brust method, thereby establishing that both DR and Brust methods lead to the formation of atomically precise Au NMs, regardless of the order of thiol addition and reduction steps. 
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  4. null (Ed.)
    Single-molecule approaches for probing the free energy of confinement for polymers in a nanopore environment are critical for the development of nanopore biosensors. We developed a laser-based nanopore heating approach to monitor the free energy profiles of such a single-molecule sensor. Using this approach, we measure the free energy profiles of two distinct polymers, polyethylene glycol and water-soluble peptides, as they interact with the nanopore sensor. Polyethylene glycol demonstrates a retention mechanism dominated by entropy with little sign of interaction with the pore, while peptides show an enthalpic mechanism, which can be attributed to physisorption to the nanopore (e.g., hydrogen bonding). To manipulate the energetics, we introduced thiolate-capped gold clusters [Au 25 (SG) 18 ] into the pore, which increases the charge and leads to additional electrostatic interactions that help dissect the contribution that enthalpy and entropy make in this modified environment. These observations provide a benchmark for optimization of single-molecule nanopore sensors. 
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  5. null (Ed.)