"Designing high-performance nighttime thermoradiative systems for harvesting energy from outer space" by Zhang et al.: comment.

The current-voltage characteristics presented by Zhang et al. in their recent work on designing thermoradiative systems overestimate the achievable power using the proposed material by several orders of magnitude.

Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles.

Optical tweezers are widely used in materials assembly1, characterization2, biomechanical force sensing3,4 and the in vivo manipulation of cells5 and organs6. The trapping force has primarily been generated through the refractive index mismatch between a trapped object and its surrounding medium. This poses a fundamental challenge for the optical trapping of low-refractive-index nanoscale objects, including nanoparticles and intracellular organelles. Here, we report a technology that employs a resonance effect to enhance the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude. This effectively bypasses the requirement of refractive index mismatch at the nanoscale. We show that under resonance conditions, highly doping lanthanide ions in NaYF4 nanocrystals makes the real part of the Clausius-Mossotti factor approach its asymptotic limit, thereby achieving a maximum optical trap stiffness of 0.086 pN µm-1 mW-1 for 23.3-nm-radius low-refractive-index (1.46) nanoparticles, that is, more than 30 times stronger than the reported value for gold nanoparticles of the same size. Our results suggest a new potential of lanthanide doping for the optical control of the refractive index of nanomaterials, developing the optical force tag for the intracellular manipulation of organelles and integrating optical tweezers with temperature sensing and laser cooling7 capabilities.

High-resolution light-activated electrochemistry on amorphous silicon-based photoelectrodes.

Light-activated electrochemistry (LAE) consists of employing a focused light beam to illuminate a semiconducting area and make it electrochemically active. Here, we show how to reduce the electrochemical spatial resolution to submicron by exploiting the short lateral diffusion of charge carriers in amorphous silicon to improve the resolution of LAE by 60 times.

Impact of metal crystallinity-related morphologies on the sensing performance of plasmonic nanohole arrays.

Plasmonic nanohole arrays for biosensing applications have attracted tremendous attention because of their flexibility in optical signature design, high multiplexing capabilities, simple optical alignment setup, and high sensitivity. The quality of the metal film, including metal crystallinity and surface roughness, plays an important role in determining the sensing performance because the interaction between free electrons in the metal and incident light is strongly influenced by the metal surface morphology. We systematically investigated the influence of metal crystallinity-related morphologies on the sensing performance of plasmonic nanohole arrays after different metal deposition processes. We utilised several non-destructive nanoscale surface characterisation techniques to perform a quantitative and comparative analysis of the Au quality of the fabricated sensor. We found empirically how the surface roughness and grain sizes influence the permittivity of the Au film and thus the sensitivity of the fabricated sensor. Finally we confirmed that the deposition conditions that provide both low surface roughness and large metal grain sizes improve the sensitivity of the plasmonic sensor.

Optical tweezers-based characterisation of gold core-satellite plasmonic nano-assemblies incorporating thermo-responsive polymers.

We report on the characterisation of the optical properties and dynamic behaviour of optically trapped single stimuli-responsive plasmonic nanoscale assemblies. Nano-assemblies consist of a core-satellite arrangement where the constituent nanoparticles are connected by the thermoresponsive polymer, poly(DEGA-co-OEGA). The optical tweezers allow the particles to be held isolated in solution and interrogated using dark-field spectroscopy. Additionally, controlling the optical trapping power provides localised heating for probing the thermal response of the nanostructures. Our results identify a number of distinct core-satellite configurations that can be stably trapped, which are verified using finite element modelling. Laser heating of the nanostructures through the trapping laser yields irreversible modification of the arrangement, as observed through the scattering spectrum. We consider which factors may be responsible for the observed behaviour in the context of the core-satellite geometry, polymer-solvent interaction, and the bonding of the nanoparticles.

Nanopore blockade sensors for ultrasensitive detection of proteins in complex biological samples.

Nanopore sensors detect individual species passing through a nanoscale pore. This experimental paradigm suffers from long analysis times at low analyte concentration and non-specific signals in complex media. These limit effectiveness of nanopore sensors for quantitative analysis. Here, we address these challenges using antibody-modified magnetic nanoparticles ((anti-PSA)-MNPs) that diffuse at zero magnetic field to capture the analyte, prostate-specific antigen (PSA). The (anti-PSA)-MNPs are magnetically driven to block an array of nanopores rather than translocate through the nanopore. Specificity is obtained by modifying nanopores with anti-PSA antibodies such that PSA molecules captured by (anti-PSA)-MNPs form an immunosandwich in the nanopore. Reversing the magnetic field removes (anti-PSA)-MNPs that have not captured PSA, limiting non-specific effects. The combined features allow detecting PSA in whole blood with a 0.8 fM detection limit. Our 'magnetic nanoparticle, nanopore blockade' concept points towards a strategy to improving nanopore biosensors for quantitative analysis of various protein and nucleic acid species.

Micropatterning of porous silicon Bragg reflectors with poly(ethylene glycol) to fabricate cell microarrays: Towards single cell sensing.

The work presented here describes the development of an optical label-free biosensor based on a porous silicon (PSi) Bragg reflector to study heterogeneity in single cells. Photolithographic patterning of a poly(ethylene glycol) (PEG) hydrogel with a photoinitiator was employed on RGD peptide-modified PSi to create micropatterns with cell adhesive and cell repellent areas. Macrophage J774 cells were incubated to form cell microarrays and single cell arrays. Moreover, cells on the microarrays were lysed osmotically with Milli-Q™ water and the infiltration of cell lysate into the porous matrix was monitored by measuring the red shift in the reflectivity. On average, the magnitude of red shift increased with the increase in the number of cells on the micropatterns. The red shift from the spots with single cells varied from spot to spot emphasizing the heterogeneous nature of the individual cells.

A rapid readout for many single plasmonic nanoparticles using dark-field microscopy and digital color analysis.

The integration of plasmonic nanoparticles into biosensors has the potential to increase the sensitivity and dynamic range of detection, through the use of single nanoparticle assays. The analysis of the localized surface plasmon resonance (LSPR) of plasmonic nanoparticles has allowed the limit of detection of biosensors to move towards single molecules. However, due to complex equipment or slow analysis times, these technologies have not been implemented for point-of-care detection. Herein, we demonstrate an advancement in LSPR analysis by presenting a technique, which utilizes an inexpensive CMOS-equipped digital camera and a dark-field microscope, that can analyse the λmax of over several thousand gold nanospheres in less than a second, without the use of a spectrometer. This improves the throughput of single particle spectral analysis by enabling more nanoparticles to be probed and in a much shorter time. This technique has been demonstrated through the detection of interleukin-6 through a core-satellite binding assay. We anticipate that this technique will aid in the development of high-throughput, multiplexed and point-of-care single nanoparticle biosensors.

Noise induced aperiodic rotations of particles trapped by a non-conservative force.

We describe a mechanism whereby random noise can play a constructive role in the manifestation of a pattern, aperiodic rotations, that would otherwise be damped by internal dynamics. The mechanism is described physically in a theoretical model of overdamped particle motion in two dimensions with symmetric damping and a non-conservative force field driven by noise. Cyclic motion only occurs as a result of stochastic noise in this system. However, the persistence of the cyclic motion is quantified by parameters associated with the non-conservative forcing. Unlike stochastic resonance or coherence resonance, where noise can play a constructive role in amplifying a signal that is otherwise below the threshold for detection, in the mechanism considered here, the signal that is detected does not exist without the noise. Moreover, the system described here is a linear system.

Using back focal plane interferometry to probe the influence of Zernike aberrations in optical tweezers.

We experimentally investigate the influence of geometric aberrations in optical tweezers using back focal plane interferometry. We found that the introduction of coma aberrations causes significant modification to the Brownian motion of the trapped particle, producing an apparent cross-coupling between the in-plane aberrated axis and the weaker propagation axis. This coupling is evidenced by the emergence of a second dominant low frequency Lorentzian feature in the position power spectral density. The effect on Brownian motion was confirmed using a secondary unaberrated probe beam, ruling out the possibility of systematic optical effects related to the detection system.

Decoupling the effects of confinement and passivation on semiconductor quantum dots.

Semiconductor (SC) quantum dots (QDs) have recently been fabricated by both chemical and plasma techniques for specific absorption and emission of light. Their optical properties are governed by the size of the QD and the chemistry of any passivation at their surface. Here, we decouple the effects of confinement and passivation by utilising DC magnetron sputtering to fabricate SC QDs in a perfluorinated polyether oil. Very high band gaps are observed for fluorinated QDs with increasing levels of quantum confinement (from 4.2 to 4.6 eV for Si, and 2.5 to 3 eV for Ge), with a shift down to 3.4 eV for Si when oxygen is introduced to the passivation layer. In contrast, the fluorinated Si QDs display a constant UV photoluminescence (3.8 eV) irrespective of size. This ability to tune the size and passivation independently opens a new opportunity to extending the use of simple semiconductor QDs.

Nonconservative dynamics of optically trapped high-aspect-ratio nanowires.

We investigate the dynamics of high-aspect-ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the dynamics reveals a broad spectral resonance of the order of kHz with peak properties that are strongly dependent on the input trapping power. A dynamical model incorporating linear restoring optical forces, a nonconservative asymmetric coupling between translational and rotational degrees of freedom, viscous drag, and white noise provides an excellent fit to experimental observations. A persistent low-frequency cyclical motion around the equilibrium trapping position, with a frequency distinct from the spectral resonance, is observed from the time series data.

Optical Manipulation and Spectroscopy Of Silicon Nanoparticles Exhibiting Dielectric Resonances.

We demonstrate that silicon (Si) nanoparticles with scattering properties exhibiting strong dielectric resonances can be successfully manipulated using optical tweezers. The large dielectric constant of Si has a distinct advantage over conventional colloidal nanoparticles in that it leads to enhanced trapping forces without the heating associated with metallic nanoparticles. Further, the spectral features of the trapped nanoparticles provide a unique marker for probing size, shape, orientation and local dielectric environment. We exploit these properties to investigate the trapping dynamics of Si nanoparticles with different dimensions ranging from 50 to 200 nm and aspect ratios between 0.4 and 2. The unique combination of spectral and trapping properties make Si nanoparticles an ideal system for delivering directed nanoscale sensing in a range of potential applications.

Enhancing Quantum Dots for Bioimaging using Advanced Surface Chemistry and Advanced Optical Microscopy: Application to Silicon Quantum Dots (SiQDs).

Fluorescence lifetime imaging microscopy is successfully demonstrated in both one- and two-photon cases with surface modified, nanocrystalline silicon quantum dots in the context of bioimaging. The technique is further demonstrated in combination with Förster resonance energy transfer studies where the color of the nanoparticles is tuned by using organic dye acceptors directly conjugated onto the nanoparticle surface.

Ultrasensitive and specific measurement of protease activity using functionalized photonic crystals.

Herein is presented a microsensor technology as a diagnostic tool for detecting specific matrix metalloproteinases (MMPs) at very low concentrations. MMP-2 and MMP-9 are detected using label free porous silicon (PSi) photonic crystals that have been made selective for a given MMP by filling the nanopores with synthetic polymeric substrates containing a peptide sequence for that MMP. Proteolytic cleavage of the peptide sequence results in a shift in wavelength of the main peak in the reflectivity spectrum of the PSi device, which is dependent on the amount of MMP present. The ability to detect picogram amounts of MMP-2 and MMP-9 released by primary retinal pigment epithelial (RPE) cells and iris pigment epithelial (IPE) cells stimulated with lipopolysaccharide (LPS) is demonstrated. It was found that both cell types secrete higher amounts of MMP-2 than MMP-9 in their stimulated state, with RPE cells producing higher amounts of MMPs than IPE cells. The microsensor performance was compared to conventional protease detection systems, including gelatin zymography and enzyme linked immunosorbent assay (ELISA). It was found that the PSi microsensors were more sensitive than gelatin zymography; PSi microsensors detected the presence of both MMP-2 and MMP-9 while zymography could only detect MMP-2. The MMP-2 and MMP-9 quantification correlated well with the ELISA. This new method of detecting protease activity shows superior performance to conventional protease assays and has the potential for translation to high-throughput multiplexed analysis.

Chemical patterning on preformed porous silicon photonic crystals: towards multiplex detection of protease activity at precise positions†Electronic supplementary information (ESI) available: SEM images, XPS result and more optical reflectivity data. See DOI: 10.1039/c4tb00281dClick here for additional data file.

Porous silicon (PSi) rugate filters modified with alkyne-terminated monolayers were chemically patterned using a combination of photolithography of photoresist and click chemistry. Two chemical functionalities were obtained by conjugating, via click reactions, ethylene glycol moieties containing two different terminal groups to discrete areas towards the exterior of a PSi rugate filter. The patterning of biological species to the functionalized surface was demonstrated through the conjugation of fluorescein isothiocyanate labelled bovine serum albumin (FITC-BSA). Fluorescence microscopy showed selective positioning of FITC-BSA at discretely functionalized areas. Meanwhile, the optical information from precisely defined positions on the patterned surface was monitored by optical reflectivity measurements. The optical measurements revealed successful step-wise chemical functionalization followed by immobilization of gelatin. Multiplex detection of protease activity from different array elements on the patterned surface was demonstrated by monitoring the blue shifts in the reflectivity spectra resulted from the digestion of gelatin by subtilisin. Precise information from both individual elements and average population was acquired. This technique is important for the development of PSi into a microarray platform for highly parallel biosensing applications, especially for cell-based assays.

Antibody modified porous silicon microparticles for the selective capture of cells.

Herein, the ability of porous silicon (PSi) particles for selectively binding to specific cells is investigated. PSi microparticles with a high reflectance band in the reflectivity profile are fabricated, and subsequently passivated and modified with antibodies via the Cu(I)-catalyzed alkyne-azide cycloaddition reaction and succimidyl activation. To demonstrate the ability of the antibody-modified PSi particles to selectively bind to one cell type over others, HeLa cells were transfected with surface epitopes fused to fluorescent proteins. The antibody-functionalized PSi particles showed good selectivity for the corresponding surface protein on HeLa cells, with no significant cross-reactivity. The results are important for the application of PSi particles in cell sensing and drug delivery.

Versatile "click chemistry" approach to functionalizing silicon quantum dots: applications toward fluorescent cellular imaging.

In this study, we describe a solution procedure for the preparation and surface modification of photostable colloidal silicon quantum dots (SiQDs) for imaging of cancer cells. Photoluminescent SiQDs were synthesized by reduction of halogenated silane precursors using a microemulsion process. It was shown that 1,8-nonadiyne molecules could be grafted onto the surface of hydrogen-terminated SiQDs via ultraviolet (UV)-promoted hydrosilylation, demonstrated by Fourier transform infrared spectroscopy (FTIR) measurements. In addition, various azide molecules were coupled onto nonadiyne-functionalized particles, rendering particles dispersible in selected polar and nonpolar solvents. The photoluminescence of functionalized SiQDs was stable against photobleaching and did not vary appreciably within biologically applicable pH and temperature ranges. To demonstrate compatibility with biological systems, water-soluble SiQDs were used for fluorescent imaging of HeLa cells. In addition, the SiQDs were shown to be non-cytotoxic at concentrations up to 240 µg/mL. The results presented herein provide good evidence for the versatility of functionalized SiQDs for fluorescent bioimaging application.

Colloidal silicon quantum dots: from preparation to the modification of self-assembled monolayers (SAMs) for bio-applications.

Concerns over possible toxicities of conventional metal-containing quantum dots have inspired growing research interests in colloidal silicon nanocrystals (SiNCs), or silicon quantum dots (SiQDs). This is related to their potential applications in a number of fields such as solar cells, optoelectronic devices and fluorescent bio-labelling agents. The past decade has seen significant progress in the understanding of fundamental physics and surface properties of silicon nanocrystals. Such understanding is based on the advances in the preparation and characterization of surface passivated colloidal silicon nanocrystals. In this critical review, we summarize recent advances in the methods of preparing high quality silicon nanocrystals and strategies for forming self-assembled monolayers (SAMs), with a focus on their bio-applications. We highlight some of the major challenges that remain, as well as lessons learnt when working with silicon nanocrystals (239 references).

Photolithographic strategy for patterning preformed, chemically modified, porous silicon photonic crystal using click chemistry.

Porous silicon (PSi) is an ideal platform for label-free biosensing, and the development of porous silicon patterning will open a pathway to the development of highly parallel PSi biochips for detecting multiple analytes. The optical response of PSi photonic crystal is determined by the changes in the effective bulk refractive index resulting from reactions/events occurring within the internal pore space. Therefore, introducing precise chemical functionalities in the pores of PSi is essential to ensure device selectivity. Here we describe the fabrication of PSi patterns that possess discrete chemical functionalities that are restricted to precise locations. The key difference to previous patterning protocols for PSi is that the entire porous material is first modified with a self-assembled monolayer of a α,ω-diyne adsorbate prior to patterning using a microfabricated titanium mask. The distal alkyne moieties in the monolayer are then amenable to further selective modification by the archetypal "click" reaction, the copper catalyzed alkyne-azide cycloaddition (CuAAC), using the titanium mask as a resist. This type of patterning is suitable for further immobilization of biological recognition elements, and presents a new platform for highly parallel PSi biosensor for multiple detections.