Chemically synthesized metal nanoparticles with morphological chiral features are known to exhibit strong circular dichroism. However, we still lack understanding of the correlation between... Show moreChemically synthesized metal nanoparticles with morphological chiral features are known to exhibit strong circular dichroism. However, we still lack understanding of the correlation between morphological and chiroptical features of plasmonic nanoparticles. To shed light on that question, single nanoparticle experiments are required. We performed photothermal circular dichroism measurements of single chiral and achiral gold nanoparticles and correlated the chiroptical response to the 3D morphology of the same nanoparticles retrieved by electron tomography. In contrast to an ensemble measurement, we show that individual particles within the ensemble display a broad distribution of strength and handedness of circular dichroism signals. Whereas obvious structural chiral features, such as helical wrinkles, translate into chiroptical ones, nanoparticles with less obvious chiral morphological features can also display strong circular dichroism signals. Interestingly, we find that even seemingly achiral nanoparticles can display large g-factors. The origin of this circular dichroism signal is discussed in terms of plasmonics and other potentially relevant factors. Show less
Circular dichroism (CD) is a standard method for the analysis of biomolecular conformation. It is very reliable when applied to molecules, but requires relatively large amounts of solution.... Show moreCircular dichroism (CD) is a standard method for the analysis of biomolecular conformation. It is very reliable when applied to molecules, but requires relatively large amounts of solution. Plasmonics offer the perspective of enhancement of CD signals, which would extend CD spectrometry to smaller amounts of molecules and to weaker chiral signals. However, plasmonic enhancement comes at the cost of additional complications: averaging over all orientations is no longer possible or reliable, linear dichroism leaks into CD signals because of experimental imperfections, scattering and its interference with the incident beam must be taken into account, and the interaction between chiral molecules and possibly chiral plasmonic structures considerably complicates the interpretation of measured signals. This Perspective aims to explore the motivations and problems of plasmonic chirality and to re-evaluate present and future solutions. Show less
Xie, M.; Liu, H.; Wan, S.; Lu, X.; Hong, D.; Du, Y.; ... ; Tian, Y. 2022
.Sensitive detection of weak acoustic signals at nanometer scale is challenging. Here, the authors present an acoustic detection system based on a single molecule as a probe, where frequency and... Show more.Sensitive detection of weak acoustic signals at nanometer scale is challenging. Here, the authors present an acoustic detection system based on a single molecule as a probe, where frequency and amplitude of acoustic vibrations can be extracted from its minute variations in distance to the surface of a plasmonic gold nanorod.Sensitive detection of local acoustic vibrations at the nanometer scale has promising potential applications involving miniaturized devices in many areas, such as geological exploration, military reconnaissance, and ultrasound imaging. However, sensitive detection of weak acoustic signals with high spatial resolution at room temperature has become a major challenge. Here, we report a nanometer-scale system for acoustic detection with a single molecule as a probe based on minute variations of its distance to the surface of a plasmonic gold nanorod. This system can extract the frequency and amplitude of acoustic vibrations with experimental and theoretical sensitivities of 10 pm Hz(-1/2) and 10 fm Hz(-1/2), respectively. This approach provides a strategy for the optical detection of acoustic waves based on molecular spectroscopy without electromagnetic interference. Moreover, such a small nano-acoustic detector with 40-nm size can be employed to monitor acoustic vibrations or read out the quantum states of nanomechanical devices. Show less
We demonstrate two-photon-excited single-mole-culefluorescence enhancement by single end-to-end self-assembledgold nanorod dimers. We employed biotinylated streptavidin asthe molecular linker,... Show moreWe demonstrate two-photon-excited single-mole-culefluorescence enhancement by single end-to-end self-assembledgold nanorod dimers. We employed biotinylated streptavidin asthe molecular linker, which connected two gold nanorods in end-to-end fashion. The typical size of streptavidin of around 5 nmseparates the gold nanorods with gaps suitable for the access offresh dyes in aqueous solution, yet small enough to give very hightwo-photonfluorescence enhancement. Simulations show thatenhancements of more than 7 orders of magnitude can be achievedfor two-photon-excitedfluorescence in the plasmonic hot spots.With such high enhancements, we successfully detect two-photon-excitedfluorescence for a common organic dye (ATTO 610) at the single-molecule, single-nanoparticle level Show less
End-to-end gold nanorod dimers provide unique plasmonic hotspots with extremely large near-field enhancements in the gaps. Thereby they are beneficial in a wide range of applications, such as... Show moreEnd-to-end gold nanorod dimers provide unique plasmonic hotspots with extremely large near-field enhancements in the gaps. Thereby they are beneficial in a wide range of applications, such as enhancing the emissions from ultra-weak emitters. For practical purposes, synthesis of gold nanorod dimers with high yield, especially on the substrates, is essential. Here, we demonstrate two controllable strategies to synthesize gold nanorod dimers based on the self-assembly of gold nanorods, either in bulk solution or on the surface of a glass substrate directly. Both methods can give a high yield of gold nanorod dimers, yet, assembling them directly on the substrate provides more flexibility in controlling the shape and size of each nanorod within the dimer. We also show that these gold nanorod dimers can be used to enhance two-photon-excited fluorescence signals at the single-molecule level. Show less
We review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in... Show moreWe review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in fluorescence, super-resolution, photothermal contrast, and interferometric scattering and briefly discuss a few of the applications. These advances have enabled the exploration of new emitters and quantum optics; the chemistry and biology of complex heterogeneous systems, nanoparticles, and plasmonics; and the detection and study of non-fluorescing and non-absorbing nano-objects. We conclude by proposing some ideas for future experiments. The field will move toward more and better signals of a broader variety of objects and toward a sharper view of the surprising complexity of the nanoscale world of single (bio-)molecules, nanoparticles, and their nano-environments. Published under an exclusive license by AIP Publishing. Show less
Magnetic imaging is a versatile tool in biological andcondensed-matter physics. Existing magnetic imaging techniqueseither require demanding experimental conditions which restrict therange of their... Show moreMagnetic imaging is a versatile tool in biological andcondensed-matter physics. Existing magnetic imaging techniqueseither require demanding experimental conditions which restrict therange of their applications or lack the spatial resolution required forsingle-particle measurements. Here, we combine photothermal (PT)microscopy with magnetic circular dichroism (MCD) to develop aversatile magnetic imaging technique using visible light. Unlike mostmagnetic imaging techniques, photothermal magnetic circulardichroism (PT MCD) microscopy works particularly well for singlenanoparticles immersed in liquids. As a proof of principle, wedemonstrate magnetic CD imaging of superparamagnetic magnetitenanoparticulate clusters immersed in microscope immersion oil. Thesensitivity of our method allowed us to probe the magnetizationcurve of single similar to 400-nm-diameter magnetite nanoparticulate clusters Show less
Detection of single molecules or particles avoids ensemble averaging and thus is able to provide a very local heterogeneous information which is not available from an ensemble measurement. Single... Show moreDetection of single molecules or particles avoids ensemble averaging and thus is able to provide a very local heterogeneous information which is not available from an ensemble measurement. Single molecules or particles can be detected based on their three characteristic optical properties, absorption, scattering and fluorescence/photoluminescence, in addition to their label-free detection. This short review focuses on our group's research towards understanding and imaging the above three kinds of optical signals from single molecules and particles. Show less
Circular dichroism (CD) is the property of chiral nanoobjects to absorb circularly polarized light of either handedness to different extents. Photothermal microscopy enables the detection of CD... Show moreCircular dichroism (CD) is the property of chiral nanoobjects to absorb circularly polarized light of either handedness to different extents. Photothermal microscopy enables the detection of CD signals with high sensitivity and provides a direct absorptive response of the samples under study. To achieve CD measurements at the single-particle level, one must reduce such artifacts as leakage of linear dichroism (LD) and residual intensity modulation. We have simulated our setup with a simple model, which allows us to tune modulation parameters to obtain a CD signal virtually free from artifacts. We demonstrate the sensitivity of our setup by measuring the very weak inherent CD signals of single gold nanospheres. We furthermore demonstrate that our method can be extended to obtain spectra of the full absorptive properties of single nanoparticles, including isotropic absorption, linear dichroism, and circular dichroism. We then investigate nominally achiral gold nanoparticles immersed in a chiral liquid. Carefully taking into account the intrinsic chirality of the particles and its change due to heat-induced reshaping, we find that the chiral liquid carvone surrounding the particle has no measurable effect on the particles' chirality, down to g-factors of 3 x 10(-4). Show less
