Biomolecular interactions on densely coated nanoparticles: a single-molecule perspective.

DNA-modified gold nanoparticles (AuNPs) play a pivotal role in bio-nanotechnology, driving advancements in bio-sensing, bio-imaging, and drug delivery. Synthetic protocols have focused on maximizing the receptor density on particles by fine-tuning chemical conditions, particularly for DNA. Despite their significance, the understanding of hybridization kinetics on functionalized AuNPs is lacking, particularly how this kinetics depends on DNA density and to what extent it varies from particle-to-particle. This study explores the molecular mechanisms of DNA hybridization on densely coated AuNPs by employing a combination of single-molecule microscopy and coarse-grained molecular dynamics simulations providing a quantification of the molecular rate constants for single particles. Our findings demonstrate that DNA receptor density and the presence of spacer strands profoundly impact association kinetics, with short spacers enhancing association rates by up to ∼15-fold. In contrast, dissociation kinetics are largely unaffected by receptor density within the studied range. Single-particle analysis directly reveals variability in hybridization kinetics, which is analyzed in terms of intra- and inter-particle heterogeneity. A coarse-grained DNA model that quantifies hybridization kinetics on densely coated surfaces further corroborates our experimental results, additionally shedding light on how transient base pairing within the DNA coating influences kinetics. This integrated approach underscores the value of single-molecule studies and simulations for understanding DNA dynamics on densely coated nanoparticle surfaces, offering guidance for designing DNA-functionalized nanoparticles in sensor applications.

Correction: Concentration and temperature dependent interactions and state diagram of dispersions of copolymer microgels.

Correction for 'Concentration and temperature dependent interactions and state diagram of dispersions of copolymer microgels' by José Ruiz-Franco et al., Soft Matter, 2023, 19, 3614-3628, https://doi.org/10.1039/D3SM00120B.

Concentration and temperature dependent interactions and state diagram of dispersions of copolymer microgels.

We investigate by means of small angle neutron scattering experiments and numerical simulations the interactions and inter-particle arrangements of concentrated dispersions of copolymer poly(N-isopropylacrylamide)-poly(ethylene glycol methyl ether methacrylate) (PNIPAM-PEGMA) microgels across the volume phase transition (VPT). The scattering data of moderately concentrated dispersions are accurately modeled at all temperatures by using a star polymer form factor and static structure factors calculated from the effective potential obtained from simulations. Interestingly, for temperatures below the VPT temperature (VPTT), the radius of gyration and blob size of the particles significantly decrease with increasing the effective packing fraction in the non-overlapping regime. This is attributed to the presence of charges in the system associated with the use of an ionic initiator in the synthesis. Simulations using the experimentally corroborated interaction potential are used to explore the state diagram in a wide range of effective packing fractions. Below and slightly above the VPTT, the system undergoes an arrest transition mainly driven by the soft repulsion between the particles. Only well above the VPTT the system is found to phase separate before arresting. Our results highlight the versatility and potential of copolymer PNIPAM-PEGMA microgels to explore different kinds of arrested states balancing attraction and repulsion by changing temperature and packing fraction.

Probing Temperature Responsivity of Microgels and Its Interplay with a Solid Surface by Super-Resolution Microscopy and Numerical Simulations.

Super-resolution microscopy has become a powerful tool to investigate the internal structure of complex colloidal and polymeric systems, such as microgels, at the nanometer scale. An interesting feature of this method is the possibility of monitoring microgel response to temperature changes in situ. However, when performing advanced microscopy experiments, interactions between the particle and the environment can be important. Often microgels are deposited on a substrate, since they have to remain still for several minutes during the experiment. This study uses direct stochastic optical reconstruction microscopy (dSTORM) and advanced coarse-grained molecular dynamics simulations to investigate how individual microgels anchored on hydrophilic and hydrophobic surfaces undergo their volume phase transition with temperature. We find that, in the presence of a hydrophilic substrate, the structure of the microgel is unperturbed and the resulting density profiles quantitatively agree with simulations performed under bulk conditions. Instead, when a hydrophobic surface is used, the microgel spreads at the interface and an interesting competition between the two hydrophobic strengths,monomer-monomer vs monomer-surface,comes into play at high temperatures. The robust agreement between experiments and simulations makes the present study a fundamental step to establish this high-resolution monitoring technique as a platform for investigating more complex systems, these being either macromolecules with peculiar internal structure or nanocomplexes where molecules of interest can be encapsulated in the microgel network and controllably released with temperature.

Link between Morphology, Structure, and Interactions of Composite Microgels.

We combine small-angle scattering experiments and simulations to investigate the internal structure and interactions of composite poly(N-isopropylacrylamide)-poly(ethylene glycol) (PNIPAM-PEG) microgels. At low temperatures the experimentally determined form factors and the simulated density profiles indicate a loose internal particle structure with an extended corona that can be modeled as a starlike object. With increasing temperature across the volumetric phase transition, the form factor develops an inflection that, using simulations, is interpreted as arising from a conformation in which PEG chains are incorporated in the interior of the PNIPAM network. This gives rise to a peculiar density profile characterized by two dense, separated regions, at odds with configurations in which the PEG chains reside on the surface of the PNIPAM core. The conformation of the PEG chains also have profound effects on the interparticle interactions: Although chains on the surface reduce the solvophobic attraction typically experienced by PNIPAM particles at high temperatures, PEG chains inside the PNIPAM network shift the onset of attractive interaction at even lower temperatures. Our results show that by tuning the morphology of the composite microgels, we can qualitatively change both their structure and their mutual interactions, opening the way to explore new collective behaviors of these objects.

i-Rheo: determining the linear viscoelastic moduli of colloidal dispersions from step-stress measurements.

We report on the application of a Fourier transform-based method, 'i-Rheo', to evaluate the linear viscoelastic moduli of hard-sphere colloidal dispersions, both in the fluid and glass states, from a direct analysis of raw step-stress (creep) experimental data. We corroborate the efficacy of i-Rheo by comparing the outputs of creep tests performed on homogenous complex fluids to conventional dynamic frequency sweeps. A similar approach is adopted for a number of colloidal suspensions over a broad range of volume fractions. For these systems, we test the limits of the method by varying the applied stress across the materials' linear and non-linear viscoelastic regimes, and we show that the best results are achieved for stress values close to the upper limit of the materials' linear viscoelastic regime, where the signal-to-noise ratio is at its highest and the non-linear phenomena have not appeared yet. We record that, the range of accessible frequencies is controlled at the higher end by the relative weight between the inertia of the instrument and the elasticity of the complex material under investigation; whereas, the lowest accessible frequency is dictated by the extent of the materials' linear viscoelastic regime. Nonetheless, despite these constrains, we confirm the effectiveness of i-Rheo for gaining valuable information on the materials' linear viscoelastic properties even from 'creep ringing' data, confirming its potency and general validity as an accurate method for determining the material's rheological behaviour for a variety of complex systems.

Different routes into the glass state for soft thermo-sensitive colloids.

We report an experimental and theoretical investigation of glass formation in soft thermo-sensitive colloids following two different routes: a gradual increase of the particle number density at constant temperature and an increase of the radius in a fixed volume at constant particle number density. Confocal microscopy experiments and the non-equilibrium self-consistent generalized Langevin equation (NE-SCGLE) theory consistently show that the two routes lead to a dynamically comparable state at sufficiently long aging times. However, experiments reveal the presence of moderate but persistent structural differences. Successive cycles of radius decrease and increase lead instead to a reproducible glass state, indicating a suitable route to obtain rejuvenation without using shear fields.