Progress and perspectives in single-molecule optical spectroscopy

Optical characterization of a single molecule complete spatial orientation using intra-molecular triplet-triplet absorption

Progress in single molecule fluorescence experiments have enabled an in-depth characterization of fluorophores, ranging from their photophysical rates to the orientation of their emission dipole moments in three dimensions. However, one crucial spatial information remains elusive: the molecule orientation relative to its emission dipole moment. One can retrieve the latter only by the use of another non-colinear transition dipole moment. We experimentally demonstrate the optical retrieval of this information for single terrylene (Tr) molecules in a 30 nm thin para-terphenyl matrix. We show, through second-order correlation measurements at varying excitation power and polarization, that Tr molecules experience an optically induced deshelving of their triplet states, mediated by two orthogonal intra-molecular triplet-triplet absorption dipole moments. We take advantage of these two transition dipole moments to retrieve the full orientation of the Tr molecule, employing a 3-level scheme for the molecule photophysics and analytical calculations for the exciting electric field distribution. This modelling approach enables us to accurately describe both varying power and polarization measurements, giving access to the molecule's photophysical rates and to its complete orientation in three dimensions. This includes the orientation of the singlet emission dipole moment in the laboratory frame, and the orientation of the molecule plane with respect to the singlet emission dipole moment.

An optofluidic antenna for enhancing the sensitivity of single-emitter measurements

Many single-molecule investigations are performed in fluidic environments, for example, to avoid unwanted consequences of contact with surfaces. Diffusion of molecules in this arrangement limits the observation time and the number of collected photons, thus, compromising studies of processes with fast or slow dynamics. Here, we introduce a planar optofluidic antenna (OFA), which enhances the fluorescence signal from molecules by about 5 times per passage, leads to about 7-fold more frequent returns to the observation volume, and significantly lengthens the diffusion time within one passage. We use single-molecule multi-parameter fluorescence detection (sm-MFD), fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) measurements to characterize our OFAs. The antenna advantages are showcased by examining both the slow (ms) and fast (50 μs) dynamics of DNA four-way (Holliday) junctions with real-time resolution. The FRET trajectories provide evidence for the absence of an intermediate conformational state and introduce an upper bound for its lifetime. The ease of implementation and compatibility with various microscopy modalities make OFAs broadly applicable to a diverse range of studies.

Future Paths in Cryogenic Single-Molecule Fluorescence Spectroscopy

In the last three decades, cryogenic single-molecule fluorescence spectroscopy has provided average-free understanding of the photophysics and of fundamental interactions at molecular scales. Furthermore, they propose original pathways and applications in the treatment and storage of quantum information. The ultranarrow lifetime-limited zero-phonon line acts as an excellent sensor to local perturbations caused either by intrinsic dynamical degrees of freedom, or by external perturbations, such as those caused by electric fields, elastic and acoustic deformations, or light-induced dynamics. Single aromatic hydrocarbon molecules, being sensitive to nanoscale probing at nanometer scales, are potential miniaturized platforms for integrated quantum photonics. In this Perspective, we look back at some of the past advances in cryogenic optical microscopy and propose some perspectives for future development.

Transport of Nanoparticles into Plants and Their Detection Methods

Nanoparticle transport into plants is an evolving field of research with diverse applications in agriculture and biotechnology. This article provides an overview of the challenges and prospects associated with the transport of nanoparticles in plants, focusing on delivery methods and the detection of nanoparticles within plant tissues. Passive and assisted delivery methods, including the use of roots and leaves as introduction sites, are discussed, along with their respective advantages and limitations. The barriers encountered in nanoparticle delivery to plants are highlighted, emphasizing the need for innovative approaches (e.g., the stem as a new recognition site) to optimize transport efficiency. In recent years, research efforts have intensified, leading to an evendeeper understanding of the intricate mechanisms governing the interaction of nanomaterials with plant tissues and cells. Investigations into the uptake pathways and translocation mechanisms within plants have revealed nuanced responses to different types of nanoparticles. Additionally, this article delves into the importance of detection methods for studying nanoparticle localization and quantification within plant tissues. Various techniques are presented as valuable tools for comprehensively understanding nanoparticle-plant interactions. The reliance on multiple detection methods for data validation is emphasized to enhance the reliability of the research findings. The future outlooks of this field are explored, including the potential use of alternative introduction sites, such as stems, and the continued development of nanoparticle formulations that improve adhesion and penetration. By addressing these challenges and fostering multidisciplinary research, the field of nanoparticle transport in plants is poised to make significant contributions to sustainable agriculture and environmental management.

Thiolation for Enhancing Photostability of Fluorophores at the Single-Molecule Level

Fluorescent probes are essential for single-molecule imaging. However, their application in biological systems is often limited by the short photobleaching lifetime. To overcome this, we developed a novel thiolation strategy for squaraine dyes. By introducing thiolation of the central cyclobutene of squaraine (thio-squaraine), we observed a ≈5-fold increase in photobleaching lifetime. Our single-molecule data analysis attributes this improvement to improved photostability resulting from thiolation. Interestingly, bulk measurements show rapid oxidation of thio-squaraine to its oxo-analogue under irradiation, giving the perception of inferior photostability. This discrepancy between bulk and single-molecule environments can be ascribed to the factors in the latter, including larger intermolecular distances and restricted mobility, which reduce the interactions between a fluorophore and reactive oxygen species produced by other fluorophores, ultimately impacting photobleaching and photoconversion rate. We demonstrate the remarkable performance of thio-squaraine probes in various imaging buffers, such as glucose oxidase with catalase (GLOX) and GLOX+trolox. We successfully employed these photostable probes for single-molecule tracking of CD56 membrane protein and monitoring mitochondria movements in live neurons. CD56 tracking revealed distinct motion states and the corresponding protein fractions. This investigation is expected to propel the development of single-molecule imaging probes, particularly in scenarios where bulk measurements show suboptimal performance.

Photothermal Microscopy and Spectroscopy with Nanomechanical Resonators

In nanomechanical photothermal absorption spectroscopy and microscopy, the measured substance becomes a part of the detection system itself, inducing a nanomechanical resonance frequency shift upon thermal relaxation. Suspended, nanometer-thin ceramic or 2D material resonators are innately highly sensitive thermal detectors of localized heat exchanges from substances on their surface or integrated into the resonator itself. Consequently, the combined nanoresonator-analyte system is a self-measuring spectrometer and microscope responding to a substance's transfer of heat over the entire spectrum for which it absorbs, according to the intensity it experiences. Limited by their own thermostatistical fluctuation phenomena, nanoresonators have demonstrated sufficient sensitivity for measuring trace analyte as well as single particles and molecules with incoherent light or focused and wide-field coherent light. They are versatile in their design, support various sampling methods-potentially including hydrated sample encapsulation-and hyphenation with other spectroscopic methods, and are capable in a wide range of applications including fingerprinting, separation science, and surface sciences.

Distinguishing Single-Metal Nanoparticles with Subdiffraction Spatial Resolution Using Variable-Polarization Fourier Transform Nonlinear Optical Microscopy

The development and use of interferometric variable-polarization Fourier transform nonlinear optical (vpFT-NLO) imaging to distinguish colloidal nanoparticles colocated within the optical diffraction limit is described. Using a collinear train of phase-stabilized pulse pairs with orthogonal electric field vectors, the polarization of nonlinear excitation fields are controllably modulated between linear, circular, and various elliptical states. Polarization modulation is achieved by precise control over the time delay separating the orthogonal pulse pairs to within hundreds of attoseconds. The resultant emission from gold nanorods is imaged to a 2D array detector and correlated to the excitation field polarization and plasmon resonance frequency by Fourier transformation. Gold nanorods with length-to-diameter aspect ratios of 2 support a longitudinal surface plasmon resonance at approximately 800 nm, which is resonant with the excitation fundamental carrier wavelength. Differences in the intrinsic linear and circular dichroism resulting from variation in their relative alignment with respect to the laboratory frame enable optical differentiation of nanorods separated within 50 nm, which is an approximate 5-fold improvement over the diffraction limit of the microscope. The experimental results are supported by analytical simulations. In addition to subdiffraction spatial resolution, the vpFT-NLO method intrinsically provides the polarization- and frequency-dependent resonance response of the nanoparticles-providing spectroscopic information content along with super-resolution imaging capabilities.

Thousand-Fold Enhancement of Photothermal Signals in Near-Critical CO2

Photothermal (PT) microscopy has shown strong promise in imaging single absorbing nano-objects in soft matter and biological systems. PT imaging at ambient conditions usually requires a high laser power for a sensitive detection, which prevents application to light-sensitive nanoparticles. In a previous study of single gold nanoparticles, we showed that the photothermal signal can be enhanced more than 1000-fold in near-critical xenon compared to that in glycerol, a typical medium for PT detection. In this report, we show that carbon dioxide (CO₂), a much cheaper gas than xenon, can enhance PT signals in a similar way. We confine near-critical CO₂ in a thin capillary which easily withstands the high near-critical pressure (around 74 bar) and facilitates sample preparation. We also demonstrate enhancement of the magnetic circular dichroism signal of single magnetite nanoparticle clusters in supercritical CO₂. We have performed COMSOL simulations to support and explain our experimental findings.

Ultraviolet Nanophotonics Enables Autofluorescence Correlation Spectroscopy on Label-Free Proteins with a Single Tryptophan

Using the ultraviolet autofluorescence of tryptophan amino acids offers fascinating perspectives to study single proteins without the drawbacks of fluorescence labeling. However, the low autofluorescence signals have so far limited the UV detection to large proteins containing several tens of tryptophan residues. This limit is not compatible with the vast majority of proteins which contain only a few tryptophans. Here we push the sensitivity of label-free ultraviolet fluorescence correlation spectroscopy (UV-FCS) down to the single tryptophan level. Our results show how the combination of nanophotonic plasmonic antennas, antioxidants, and background reduction techniques can improve the signal-to-background ratio by over an order of magnitude and enable UV-FCS on thermonuclease proteins with a single tryptophan residue. This sensitivity breakthrough unlocks the applicability of UV-FCS technique to a broad library of label-free proteins.