Plasmonic Nanosensors Reveal a Height Dependence of MinDE Protein Oscillations on Membrane Features

Self-organized spatial targeting of contractile actomyosin rings for synthetic cell division

A key challenge for bottom-up synthetic biology is engineering a minimal module for self-division of synthetic cells. Actin-based cytokinetic rings are considered a promising structure to produce the forces required for the controlled excision of cell-like compartments such as giant unilamellar vesicles (GUVs). Despite prior demonstrations of actin ring targeting to GUV membranes and myosin-induced constriction, large-scale vesicle deformation has been precluded due to the lacking spatial control of these contractile structures. Here we show the combined reconstitution of actomyosin rings and the bacterial MinDE protein system within GUVs. Incorporating this spatial positioning tool, able to induce active transport of membrane-attached diffusible molecules, yields self-organized equatorial assembly of actomyosin rings in vesicles. Remarkably, the synergistic effect of Min oscillations and the contractility of actomyosin bundles induces mid-vesicle deformations and vesicle blebbing. Our system showcases how functional machineries from various organisms may be combined in vitro, leading to the emergence of functionalities towards a synthetic division system.

Optical dark-field spectroscopy of single plasmonic nanoparticles for molecular biosciences

An ideal sensor capable of quantifying analytes in minuscule sample volumes represents a significant technological advancement. Plasmonic nanoparticles integrated with optical dark-field spectroscopy have reached this capability, demonstrating versatility and expanding applicability across in vitro and in vivo subjects. This review underscores the applicability of optical dark-field spectroscopy with single plasmonic nanoparticles to elucidate a wide range of biomolecular characteristics, including binding constants, molecular dynamics, distances, and forces, as well as recording cell communication signals. Perspectives highlight the potential for the development of implantable nanosensors for metabolite detection in animal models, illustrating the technique's efficacy without the need for labeling molecules. In summary, this review aims to consolidate knowledge of this adaptable and robust technique for decoding molecular biological phenomena within the nano- and bio-scientific community.

Nonlocal Optical Response of Particle Plasmons in Single Gold Nanorods

The investigation of particle plasmons in metal nanoparticles has predominantly relied on local optical response approximations. However, as the nanoparticle size approaches the average distance of electrons to the metal surface, mesoscopic effects such as size-dependent plasmon line width broadening and resonance energy blue shifts are expected to become observable. In this work, we compared the experimental spectral characteristics with simulated values obtained by using a generalized nonlocal optical response theory-based local analogue model. Our results show that the nonlocal plasmon damping effects in single nanoparticles are less pronounced than those observed in plasmon-coupled systems. Furthermore, our research demonstrates that single-particle dark-field spectroscopy is an effective tool for investigating the nonlocal optical response of particle plasmons in single nanoparticles. These results have important implications for the rational design of novel nanophotonic devices.

Plasmonic Nanomaterials in Dark Field Sensing Systems

Plasma nanoparticles offer promise in data storage, biosensing, optical imaging, photoelectric integration, etc. This review highlights the local surface plasmon resonance (LSPR) excitation mechanism of plasmonic nanoprobes and its critical significance in the control of dark-field sensing, as well as three main sensing strategies based on plasmonic nanomaterial dielectric environment modification, electromagnetic coupling, and charge transfer. This review then describes the component materials of plasmonic nanoprobes based on gold, silver, and other noble metals, as well as their applications. According to this summary, researchers raised the LSPR performance of composite plasmonic nanomaterials by combining noble metals with other metals or oxides and using them in process analysis and quantitative detection.

Multiple solution solving in plasmon sensing by deep learning: determination of layer refractive index and thickness in one experiment: comment

In a recent Letter [Opt. Lett.46, 5667 (2021)10.1364/OL.444442], Du et al. proposed a deep learning method for determining the refractive index (n) and thickness (d) of the surface layer on nanoparticles in a single-particle plasmon sensing experiment. This comment highlights the methodological issues arising in that Letter.

Revisiting the plasmon radiation damping of gold nanorods

Noble metal nanoparticles have been utilized for a vast amount of optical applications. For applications that use metal nanoparticles as nanosensors and for optical labeling, higher radiative efficiency is preferred. To get a deeper knowledge about the radiation damping of noble metal nanoparticles, we used gold nanorods with different geometry factors (aspect ratios) as the model system to study. We investigated theoretically how the radiation damping of a nanorod depends on the material, and shape of the particle. Surprisingly, a simple analytical equation describes radiation damping very accurately and allows the disentanglement of the maximal radiation damping parameter for gold nanorods with resonance energy E_res around 1.81 eV (685 nm). We found very good agreement with theoretical predictions and experimental data obtained by single-particle spectroscopy. Our results and approaches may pave the way for designing and optimizing gold nanostructures with higher optical signal and better sensing performance.

Multiplexed detection of heavy metal ions by single plasmonic nanosensors

Detection of multiple analytes simultaneously in small liquid samples with high efficiency and precision is highly important to the fields like water quality monitoring. In this letter, we present a multiplexed nanosensors with position-encoded aptamer functionalized gold nanorods for heavy metal ions detection. The individual gold nanorods respond specifically to two different heavy metal ions (Pb²⁺ and Hg²⁺) with a spectral shift in the scattering spectrum. We used a home-built spectral imaging dark-field microscope to measure the response of thousands of single plasmonic nanosensors with relatively high time resolution and precision. To explore the performance and limit of detection (LOD) of our nanosensor and setup, we recorded the concentration-dependent response of our position-encoded nanosensors with a series of mixture solutions that contain different concentrations of Hg²⁺ and Pb²⁺ ions. The LOD levels of our system are around 5 nM for Pb²⁺ ions and 1 nM for Hg²⁺ ions. Our method and results demostrate the nanomolar sensitivity and the potential to detect more different heavy metal ions.

Single Out-of-Resonance Dielectric Nanoparticles as Molecular Sensors

Light scattering from single nanoparticles and nanostructures is a commonly used readout method for nanosensors. Increasing the spectral sensitivity of resonant nanosensors to changes in their local surrounding has been the focus of many studies. Switching from spectral to intensity monitoring allows one to investigate nonresonant or out-of-resonance dielectric nanoparticles. Here, we systematically compared such dielectric silica nanoparticles with plasmonic gold nanorods by deriving analytical expressions and by performing experiments. The experiments show a similar sensitivity for the detection of an adsorbate layer for both particle types, which is in good agreement with theory. The flat spectral response of dielectric silica nanoparticles simplifies the choice of illumination wavelength. Furthermore, such dielectric nanoparticles can be made from many oxides, polymers, and even biological assemblies, broadening the choice of materials for the nanosensor.

Intensity-Based Single Particle Plasmon Sensing

Plasmon sensors respond to local changes of their surrounding environment with a shift in their resonance wavelength. This response is usually detected by measuring light scattering spectra to determine the resonance wavelength. However, single wavelength detection has become increasingly important because it simplifies the setup, increases speed, and improves statistics. Therefore, we investigated theoretically how the sensitivity toward such single wavelength scattering intensity changes depend on the material and shape of the plasmonic sensor. Surprisingly, simple equations describe this intensity sensitivity very accurately and allow us to distinguish the various contributions: Rayleigh scattering, dielectric contrast, plasmon shift, and frequency-dependent plasmon bulk damping. We find very good agreement of theoretical predictions and experimental data obtained by single particle spectroscopy.

Investigation of electron transfer between single plasmon and graphene by dark field spectroscopy

We investigated the electron transfer time between single plasmonic gold nanoparticles and graphene with our home-build spectral imaging dark-field microscope. The process of electron transfer is supposed to be shuttling of hot electrons on the nanoparticle-graphene interface, resulting in a slight broadening of the scattering spectrum. For detecting the minor spectrum broadening, we firstly characterized our setup systematically and then calibrated its intrinsic error. We found the mechanism of a common but normally neglected setup error, scattering spectrum broadening, which is caused by the bandwidth of the incident light and could exist in most fast dark-field microscopy setups. We corrected the linewidth of plasmon scattering spectra theoretically by both numerical and analytical solution, and then realized it experimentally by tuning the bandwidth of the incident light. After calibration, we revisited scattering spectra of 700 small aspect ratio nanorods on glass and monolayer graphene revealing a typical 14.3 meV linewidth broadening. Furthermore, we measured four other kinds of gold nanoparticles on glass, mono- and bilayer graphene for deeper understanding of the electron transfer. A common linewidth broadening is found for each kind of particle agreeing well with previous theory. However, an unconventional linewidth narrowing is also discovered for big particles whose resonance wavelength is close to the near infrared region. It implies a competitive mechanism in the electron transfer process which could not only increase the damping of small particles, causing a linewidth broadening, but also simplify the electric field pattern for big particles, leading to a linewidth narrowing, according to our Mie theory simulation.

Mass-sensitive particle tracking to elucidate the membrane-associated MinDE reaction cycle

In spite of their great importance in biology, methods providing access to spontaneous molecular interactions with and on biological membranes have been sparse. The recent advent of mass photometry to quantify mass distributions of unlabeled biomolecules landing on surfaces raised hopes that this approach could be transferred to membranes. Here, by introducing a new interferometric scattering (iSCAT) image processing and analysis strategy adapted to diffusing particles, we enable mass-sensitive particle tracking (MSPT) of single unlabeled biomolecules on a supported lipid bilayer. We applied this approach to the highly nonlinear reaction cycles underlying MinDE protein self-organization. MSPT allowed us to determine the stoichiometry and turnover of individual membrane-bound MinD/MinDE protein complexes and to quantify their size-dependent diffusion. This study demonstrates the potential of MSPT to enhance our quantitative understanding of membrane-associated biological systems.

Biologically interfaced nanoplasmonic sensors

Understanding biointerfacial processes is crucial in various fields across fundamental and applied biology, but performing quantitative studies via conventional characterization techniques remains challenging due to instrumentation as well as analytical complexities and limitations. In order to accelerate translational research and address current challenges in healthcare and medicine, there is an outstanding need to develop surface-sensitive technologies with advanced measurement capabilities. Along this line, nanoplasmonic sensing has emerged as a powerful tool to quantitatively study biointerfacial processes owing to its high spatial resolution at the nanoscale. Consequently, the development of robust biological interfacing strategies becomes imperative to maximize its characterization potential. This review will highlight and discuss the critical role of biological interfacing within the context of constructing nanoplasmonic sensing platforms for biointerfacial science applications. Apart from paving the way for the development of highly surface-sensitive characterization tools that will spur fundamental biological interaction studies and improve the overall understanding of biological processes, the basic principles behind biointerfacing strategies presented in this review are also applicable to other fields that involve an interface between an inorganic material and a biological system.

Plasmonic Nanosensors for the Label-Free Imaging of Dynamic Protein Patterns

We introduce a new approach to monitor the dynamics and spatial patterns of biological molecular assemblies. Our molecular imaging method relies on plasmonic gold nanoparticles as point-like detectors and requires no labeling of the molecules. We show spatial resolution of up to 5 μm and 30 ms temporal resolution, which is comparable to wide-field fluorescence microscopy, while requiring only readily available gold nanoparticles and a dark-field optical microscope. We demonstrate the method on MinDE proteins attaching to and detaching from lipid membranes of different composition for 24 h. We foresee our new imaging method as an indispensable tool in advanced molecular biology and biophysics laboratories around the world.

Plasmonic thermochromism based on a reversible redox reaction of Ag+/Ag on Au nanorods

Reversible redox reaction-based thermochromism using plasmonic nanocrystals is challenging due to the requirements set based on the complexity of the reaction system where the oxidizing and reducing agents must not interfere with each other, and both should possess temperature sensitivity. Herein, we demonstrate plasmonic thermochromism based on a reversible redox reaction of Ag+/Ag on Au nanorods (AuNRs) by incorporating temperature-sensitive reducing and oxidizing agents into the same system. The competition between reduction and oxidation is solely dependent on temperature. When the temperature is above (below) the transition temperature, the reduction of Ag+ (oxidation of Ag) dominates on the surface of AuNRs, and the thermochromic nanostructure solution appears green (red). An experimental study on the mechanism reveals that HOCl produced at low concentrations by H2O2 is the source of the observed temperature dependence of the Ag oxidation. Rationally tuning the transition temperature in a range from 27 to 40 °C can be realized by changing the concentration of some key chemical compounds in the solution. The thermochromic solution can be standalone-functional within multiple cycles of heating and cooling and long-term storage without any additional reagents. Our study provides new insight into plasmonic thermochromism and may pave the way for fabricating smart thermochromic materials.

Plasmonic Nanoparticle-Interfaced Lipid Bilayer Membranes

Plasmonic nanoparticles are widely exploited in diverse bioapplications ranging from therapeutics to biosensing and biocomputing because of their strong and tunable light-matter interactions, facile and versatile chemical/biological ligand modifications, and biocompatibility. With the rapid growth of nanobiotechnology, understanding dynamic interactions between nanoparticles and biological systems at the molecular or single-particle level is becoming increasingly important for interrogating biological systems with functional nanostructures and for developing nanoparticle-based biosensors and therapeutic agents. Therefore, significant efforts have been devoted to precisely design and create nano-bio interfaces by manipulating the nanoparticles' size, shape, and surface ligand interactions with complex biological systems to maximize their performance and avoid unwanted responses, such as their agglomeration and cytotoxicity. However, investigating physicochemical interactions at the nano-bio interfaces in a quantitative and controllable manner remains challenging, as the interfaces involve highly complex networks between nanoparticles, biomolecules, and cells across multiple scales, each with a myriad of different chemical and biological interactions. A lipid bilayer is a membrane made of two layers of lipid molecules that forms a barrier around cells and plays structural and functional roles in diverse biological processes because they incorporate and present functional molecules (such as membrane proteins) with lateral fluidity. Plasmonic nanoparticles conjugated on lipid membranes provide reliable analytical labels and functional moieties that allow for studying and manipulating interactions between nanoparticles and molecules with single-particle resolution; they also serve as efficient tools for applying optical, mechanical, and thermal stimuli to biological systems, which stem from plasmonic properties. Recently, new opportunities have emerged by interfacing nanoparticle-modified lipid bilayers (NLBs) with complex systems such as molecular circuits and living systems. In this Account, we briefly review how plasmonic properties can be beneficially harnessed on lipid bilayer membranes to investigate the structures and functions of cellular membranes and to develop new platforms for biomedical applications. In particular, we discuss the versatility of supported lipid bilayers (SLBs), which are planar lipid bilayers on hydrophilic substrates, as dynamic biomaterials that provide lateral fluidity and cell membrane-like environments. We then summarize our efforts to create a quantitative analytical platform utilizing nanoparticles as active building blocks and SLBs as integrative substrates. Through this bottom-up approach, various functionalized nanoparticles have been introduced onto lipid bilayers to render nanoparticle-nanoparticle, nanoparticle-lipid bilayer, and biomolecule-lipid bilayer interfaces programmable. Our system provides a new class of tools for studying thermodynamics and kinetics in complex networks of nanostructures and for realizing unique applications in biosensing and biocomputing.

Structural and mechanistic insights into the interaction of the circadian transcription factor BMAL1 with the KIX domain of the CREB-binding protein

The mammalian CLOCK:BMAL1 transcription factor complex and its coactivators CREB-binding protein (CBP)/p300 and mixed-lineage leukemia 1 (MLL1) critically regulate circadian transcription and chromatin modification. Circadian oscillations are regulated by interactions of BMAL1's C-terminal transactivation domain (TAD) with the KIX domain of CBP/p300 (activating) and with the clock protein CRY1 (repressing) as well as by the BMAL1 G-region preceding the TAD. Circadian acetylation of Lys⁵³⁷ within the G-region enhances repressive BMAL1-TAD-CRY1 interactions. Here, we characterized the interaction of the CBP-KIX domain with BMAL1 proteins, including the BMAL1-TAD, parts of the G-region, and Lys⁵³⁷ Tethering the small compound 1-10 in the MLL-binding pocket of the CBP-KIX domain weakened BMAL1 binding, and MLL1-bound KIX did not form a ternary complex with BMAL1, indicating that the MLL-binding pocket is important for KIX-BMAL1 interactions. Small-angle X-ray scattering (SAXS) models of BMAL1 and BMAL1:KIX complexes revealed that the N-terminal BMAL1 G-region including Lys⁵³⁷ forms elongated extensions emerging from the bulkier BMAL1-TAD:KIX core complex. Fitting high-resolution KIX domain structures into the SAXS-derived envelopes suggested that the G-region emerges near the MLL-binding pocket, further supporting a role of this pocket in BMAL1 binding. Additionally, mutations in the second CREB-pKID/c-Myb-binding pocket of the KIX domain moderately impacted BMAL1 binding. The BMAL1(K537Q) mutation mimicking Lys⁵³⁷ acetylation, however, did not affect the KIX-binding affinity, in contrast to its enhancing effect on CRY1 binding. Our results significantly advance the mechanistic understanding of the protein interaction networks controlling CLOCK:BMAL1- and CBP-dependent gene regulation in the mammalian circadian clock.