Label-free and real-time monitoring of single cell attachment on template-stripped plasmonic nano-holes

Plasmonic Biosensing for Label-Free Detection of Two Hallmarks of Cancer Cells: Cell-Matrix Interaction and Cell Division

Two key features of cancer cells are sustained proliferation and invasion, which is preceded by a modification of the adhesion properties to the extracellular matrix. Currently, fluorescence-based techniques are mainly used to detect these processes, including flow cytometry and fluorescence resonance energy transfer (FRET) microscopy. We have previously described a simple, fast and label-free method based on a gold nanohole array biosensor to detect the spectral response of single cells, which is highly dependent on the actin cortex. Here we used this biosensor to study two cellular processes where configuration of the actin cortex plays an essential role: cell cycle and cell–matrix adhesion. Colorectal cancer cells were maintained in culture under different conditions to obtain cells stopped either in G0/G1 (resting cells/cells at the initial steps of cell growth) or G2 (cells undergoing division) phases of the cell cycle. Data from the nanohole array biosensor showed an ability to discriminate between both cell populations. Additionally, cancer cells were monitored with the biosensor during the first 60 min after cells were deposited onto a biosensor coated with fibronectin, an extracellular matrix protein. Spectral changes were detected in the first 20 min and increased over time as the cell–biosensor contact surface increased. Our data show that the nanohole array biosensor provides a label-free and real-time procedure to detect cells undergoing division or changes in cell–matrix interaction in both clinical and research settings.

A microfluidic device enabling deterministic single cell trapping and release

Successful single-cell isolation is a pivotal technique for subsequent biological and chemical analysis of single cells. Although significant advances have been made in single-cell isolation and analysis techniques, most passive microfluidic devices cannot deterministically release trapped cells for further analysis. In this paper, we present a novel microfluidic device that can achieve high-efficiency cell trapping, which can then be released in a deterministic order. The device contains an array of trapping sites, a main channel, a trigger channel, and an air channel. Two types of capillary valves are configured along the channels. As these capillary valves can be automatically opened in a predefined pattern, the incoming cells can be spontaneously and sequentially trapped into separate trapping sites. After trapping, the individual trapped cells can be released from their sites in a last-trapped-first-released manner by applying pressure from the trigger channel to counteract against the pressure from the main channel. The theoretical model of the trapping and release flow field is established respectively to describe the conditions required for trapping and release. Experiments using MCF-7 cells demonstrated the capability of our device for deterministic single cell trapping and release. We envision that our method constitutes a useful sample preparation platform for single cell analysis.

Refractive Index Sensing for Measuring Single Cell Growth

Accessing cell growth on adhesive substrates is critical for identifying biophysical properties of cells and their therapeutic response to drug therapies. However, optical techniques have low sensitivity, and their reliability varies with cell type, whereas microfluidic technologies rely on cell suspension. In this paper, we introduced a plasmonic functional assay platform that can precisely measure cell weight and the dynamic change in real-time for adherent cells. Possessing this ability, our platform can determine growth rates of individual cells within only 10 min to map the growth profile of populations in short time intervals. The platform could successfully determine heterogeneity within the growth profile of populations and assess subpopulations exhibiting distinct growth profiles. As a proof of principle, we investigated the growth profile of MCF-7 cells and the effect of two intracellular metabolisms critical for their proliferation. We first investigated the negative effect of serum starvation on cell growth. We then studied ornithine decarboxylase (ODC) activity, a key enzyme which is involved in proliferation, and degraded under low osmolarity that inhibits cell growth. We successfully determined the significant distinction between growth profiles of MCF-7 cells and their ODC-overproducing variants that possess strong resistance to the negative effects of low osmolarity. We also demonstrated that an exogenous parameter, putrescine, could rescue cells from ODC inhibition under hypoosmotic conditions. In addition to the ability of accessing intracellular activities through ex vivo measurements, our platform could also determine therapeutic behaviors of cancer cells in response to drug treatments. Here, we investigated difluoromethylornithine (DFMO), which has antitumor effects on MCF-7 cells by inhibiting ODC activity. We successfully demonstrated the susceptibility of MCF-7 cells to such drug treatment, while its DFMO-resistant subpopulation could survive in the presence of this antigrowth agent. By rapidly determining cell growth kinetics in small samples, our plasmonic platform may be of broad use to basic research and clinical applications.

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

Actuated plasmonic nanohole arrays for sensing and optical spectroscopy applications

Herein, we report a new approach to rapidly actuate the plasmonic characteristics of thin gold films perforated with nanohole arrays that are coupled with arrays of gold nanoparticles. The near-field interaction between the localized and propagating surface plasmon modes supported by the structure was actively modulated by changing the distance between the nanoholes and nanoparticles and varying the refractive index symmetry of the structure. This approach was applied by using a thin responsive hydrogel cushion, which swelled and collapsed by a temperature stimulus. The detailed experimental study of the changes and interplay of localized and propagating surface plasmons was complemented by numerical simulations. We demonstrate that the interrogation and excitation of the optical resonance to these modes allow the label-free SPR observation of the binding of biomolecules, and is applicable for in situ SERS studies of low molecular weight molecules attached in the gap between the nanoholes and nanoparticles.

On-chip integrated optical stretching and electrorotation enabling single-cell biophysical analysis

Cells have different intrinsic markers such as mechanical and electrical properties, which may be used as specific characteristics. Here, we present a microfluidic chip configured with two opposing optical fibers and four 3D electrodes for multiphysical parameter measurement. The chip leverages optical fibers to capture and stretch a single cell and uses 3D electrodes to achieve rotation of the single cell. According to the stretching deformation and rotation spectrum, the mechanical and dielectric properties can be extracted. We provided proof of concept by testing five types of cells (HeLa, A549, HepaRG, MCF7 and MCF10A) and determined five biophysical parameters, namely, shear modulus, steady-state viscosity, and relaxation time from the stretching deformation and area-specific membrane capacitance and cytoplasm conductivity from the rotation spectra. We showed the potential of the chip in cancer research by observing subtle changes in the cellular properties of transforming growth factor beta 1 (TGF-β1)-induced epithelial-mesenchymal transition (EMT) A549 cells. The new chip provides a microfluidic platform capable of multiparameter characterization of single cells, which can play an important role in the field of single-cell research.

Wavelength-scanning surface plasmon resonance microscopy: A novel tool for real time sensing of cell-substrate interactions

This paper, for the first time, presents a wavelength-scanning surface plasmon resonance microscope (WS-SPRM) as a label-free biosensor capable of measuring cell-substrate interaction. The approach utilized a liquid crystal tunable filter (LCTF) as a fast and flexible wavelength-scanning device that can implement a wavelength-scanning and SPR imaging cycle within 1 s. The system was verified by monitoring the dynamics of cellular processes including cell detachment and electroporation of individual cells. It was found that the WS-SPRM presented better performance than the intensity-based SPRM (I-SPRM) in the imaging of cell adhesion. The results also indicated that the WS-SPRM exhibited a larger dynamic range in monitoring cell electroporation than that of I-SPRM. In summary, the developed WS-SPRM in this study provides a promising technique for real-time monitoring of cell-substrate interaction.

Advances in Surface Plasmon Resonance Imaging and Microscopy and Their Biological Applications

Surface plasmon resonance microscopy and imaging are optical methods that enable observation and quantification of interactions of nano- and microscale objects near a metal surface in a temporally and spatially resolved manner. This review describes the principles of surface plasmon resonance microscopy and imaging and discusses recent advances in these methods, in particular, in optical platforms and functional coatings. In addition, the biological applications of these methods are reviewed. These include the detection of a broad variety of analytes (nucleic acids, proteins, bacteria), the investigation of biological systems (bacteria and cells), and biomolecular interactions (drug-receptor, protein-protein, protein-DNA, protein-cell).

Nanohole array plasmonic biosensors: Emerging point-of-care applications

Point-of-care (POC) applications have expanded hugely in recent years and is likely to continue, with an aim to deliver cheap, portable, and reliable devices to meet the demands of healthcare industry. POC devices are designed, prototyped, and assembled using numerous strategies but the key essential features that biosensing devices require are: (1) sensitivity, (2) selectivity, (3) specificity, (4) repeatability, and (5) good limit of detection. Overall the fabrication and commercialization of the nanohole array (NHA) setup to the outside world still remains a challenge. Here, we review the various methods of NHA fabrication, the design criteria, the geometrical features, the effects of surface plasmon resonance (SPR) on sensing as well as current state-of-the-art of existing NHA sensors. This review also provides easy-to-understand examples of NHA-based POC biosensing applications, its current status, challenges, and future prospects.

The change of synovial fluid proteome in rabbit surgery-induced model of knee osteoarthritis

The aims of this study were to explore the change of synovial fluid (SF) proteome in a knee osteoarthritis (KOA) rabbit model, and to provide a new target for the treatment of knee osteoarthritis at the proteomic level. Sixteen New Zealand rabbits were randomly and equally divided into two groups. Group A rabbits were subjected to right anterior cruciate ligament transection (ACLT), while group B rabbits were subjected to sham ACLT. Six weeks later, the proteomes of knee joint SF from group A and B rabbits were analyzed using a label-free quantitative proteomic analysis method. We extracted 944 relevant items from GO BlastGO2 for the 23 proteins differentially expressed between the two groups. The final annotation results were 23 protein sequences annotated by 462 GO items. According to the KEGG gene database of rabbit protein sequences, as well as annotation of the KO numbers of homologous/similar proteins to the relevant 64 KEGG pathways, we extracted the sequences of 16 significantly differently expressed proteins among the relevant 64 KEGG messages/metabolism pathways. These included adiponectin, pyruvate kinase, bisphosphoglycerate mutase, HtpG/heat shock proteins, hemoglobin subunit alpha-1 2, VCP (CDC48), 14-3-3 protein beta/theta/zeta, and ferritin heavy chain, whose levels were decreased in group A. The other proteins were fibrinogen alpha/beta/gamma chain, carboxylesterase 2, paraoxonase/arylesterase 1, apolipoprotein A-I, immunoglobulin heavy chain, and transferrin, whose levels were increased in group B. The identified differentially expressed proteins indicate the change of SF proteomic expression in KOA and may provide protein targets for treating this condition.

Label-Free Optofluidic Nanobiosensor Enables Real-Time Analysis of Single-Cell Cytokine Secretion

Single-cell analysis of cytokine secretion is essential to understand the heterogeneity of cellular functionalities and develop novel therapies for multiple diseases. Unraveling the dynamic secretion process at single-cell resolution reveals the real-time functional status of individual cells. Fluorescent and colorimetric-based methodologies require tedious molecular labeling that brings inevitable interferences with cell integrity and compromises the temporal resolution. An innovative label-free optofluidic nanoplasmonic biosensor is introduced for single-cell analysis in real time. The nanobiosensor incorporates a novel design of a multifunctional microfluidic system with small volume microchamber and regulation channels for reliable monitoring of cytokine secretion from individual cells for hours. Different interleukin-2 secretion profiles are detected and distinguished from single lymphoma cells. The sensor configuration combined with optical spectroscopic imaging further allows us to determine the spatial single-cell secretion fingerprints in real time. This new biosensor system is anticipated to be a powerful tool to characterize single-cell signaling for basic and clinical research.