Amplitude and phase images of cellular structures with a scanning surface plasmon microscope

Label-free and dynamic monitoring of cell evolutions using wavelength-multiplexing surface plasmon resonance holographic microscopy

Dynamic characterizations of intracellular variations and cell-substrate interactions under different external environments are critical to study cell behaviors and exploring biological applications. However, techniques that are capable of dynamically and simultaneously measuring multiple parameters of living cells in a wide-field manner have rarely been reported. Here, we present a wavelength-multiplexing surface plasmon resonance holographic microscopy which allows wide-field, simultaneous, and dynamic measurements of cell parameters, including cell-substrate distance and cytoplasm refractive index (RI). We use two lasers of 632.8 nm and 690 nm as light sources. Two beam splitters are employed in the optical setup to separately adjust the incident angle of two light beams. Then, surface plasmon resonance (SPR) can be excited for each wavelength under SPR angles. We demonstrate the advances of the proposed apparatus by systematically studying the cell responses to osmotic pressure stimuli from the environmental medium at the cell-substrate interface. The SPR phase distributions of the cell are firstly mapped at two wavelengths, then the cell-substrate distance and cytoplasm RI are retrieved using a demodulation method. Based on phase response differences between two wavelengths and monotonic changes of SPR phase with cell parameters, cell-substrate distance, and cytoplasm RI can be determined simultaneously using an inverse algorithm. This work affords a new optical measurement technique to dynamically characterize cell evolutions and investigate cell properties in various cellular activities. It may become a useful tool in the bio-medical and bio-monitoring areas.

Recent advances in surface plasmon resonance imaging and biological applications

Surface Plasmon Resonance Imaging (SPRI) is a robust technique for visualizing refractive index changes, which enables researchers to observe interactions between nanoscale objects in an imaging manner. In the past period, scholars have been attracted by the Prism-Coupled and Non-prism Coupled configurations of SPRI and have published numerous experimental results. This review describes the principle of SPRI and discusses recent developments in Prism-Coupled and Non-prism Coupled SPRI techniques in detail, respectively. And then, major advances in biological applications of SPRI are reviewed, including four sub-fields (cells, viruses, bacteria, exosomes, and biomolecules). The purpose is to briefly summarize the recent advances of SPRI and provide an outlook on the development of SPRI in various fields.

Performance Analysis of Non-Interferometry Based Surface Plasmon Resonance Microscopes

Surface plasmon microscopy has been of interest to the science and engineering community and has been utilized in broad aspects of applications and studies, including biochemical sensing and biomolecular binding kinetics. The benefits of surface plasmon microscopy include label-free detection, high sensitivity, and quantitative measurements. Here, a theoretical framework to analyze and compare several non-interferometric surface plasmon microscopes is proposed. The scope of the study is to (1) identify the strengths and weaknesses in each surface plasmon microscopes reported in the literature; (2) quantify their performance in terms of spatial imaging resolution, imaging contrast, sensitivity, and measurement accuracy for quantitative and non-quantitative imaging modes of the microscopes. Six types of non-interferometric microscopes were included in this study: annulus aperture scanning, half annulus aperture scanning, single-point scanning, double-point scanning, single-point scanning, at 45 degrees azimuthal angle, and double-point scanning at 45 degrees azimuthal angle. For non-quantitative imaging, there is a substantial tradeoff between the image contrast and the spatial resolution. For the quantitative imaging, the half annulus aperture provided the highest sensitivity of 127.058 rad/μm² RIU⁻¹, followed by the full annulus aperture of 126.318 rad/μm² RIU⁻¹. There is a clear tradeoff between spatial resolution and sensitivity. The annulus aperture and half annulus aperture had an optimal resolution, sensitivity, and crosstalk compared to the other non-interferometric surface plasmon resonance microscopes. The resolution depends strongly on the propagation length of the surface plasmons rather than the numerical aperture of the objective lens. For imaging and sensing purposes, the recommended microfluidic channel size and protein stamping size for surface plasmon resonance experiments is at least 25 μm for accurate plasmonic measurements.

Enhanced surface plasmon microscopy based on multi-channel spatial light switching for label-free neuronal imaging

In this paper, we have investigated multi-channel switching of light incidence in multiple directions to improve image clarity in surface plasmon microscopy (SPM) for robust and consistent imaging performance regardless of the pattern geometry and shape. Multi-channel light switching in SPM allows significant reduction of adverse scattering effects by surface plasmon (SP). For proof of concept, an eight-channel spatially switched SPM (ssSPM) system has been set up. The results with reference objects including square arrays and Siemens stars experimentally confirm much improved images with ssSPM by reducing the artifacts of SP scattering significantly. On a quantitative basis, contrast analysis preformed with square arrays shows image contrast enhanced by more than three times over conventional SPM. Three image reconstruction algorithms were evaluated for optimal image acquisition. It is suggested that averaging combined with minimum-filtering produces the highest resolution. ssSPM was applied to label-free imaging of primary neuron cultures and shown to present enhanced images with clarity far better than conventional SPM.

Nanometer-Resolved Mapping of Cell-Substrate Distances of Contracting Cardiomyocytes Using Surface Plasmon Resonance Microscopy

It has been shown that quantitative measurements of the cell-substrate distance of steady cells are possible with scanning surface plasmon resonance microscopy setups in combination with an angle resolved analysis. However, the accuracy of the determined cell-substrate distances as well as the capabilities for the investigation of cell dynamics remained limited due to the assumption of a homogeneous refractive index of the cytosol. Strong spatial or temporal deviations between the local refractive index and the average value can result in errors in the calculated cell-substrate distance of around 100 nm, while the average accuracy was determined to 37 nm. Here, we present a combination of acquisition and analysis techniques that enables the measurement of the cell-substrate distance of contractile cells as well as the study of intracellular processes through changes in the refractive index at the diffraction limit. By decoupling the measurement of the cell-substrate distance and the refractive index of the cytoplasm, we could increase the accuracy of the distance measurement on average by a factor of 25 reaching 1.5 nm under ideal conditions. We show a temporal and spatial mapping of changes in the refractive index and the cell-substrate distance which strongly correlate with the action potentials and reconstruct the three-dimensional profile of the basal cell membrane and its dynamics, while we reached an actual measurement accuracy of 2.3 nm.

Hilbert transform-based single-shot plasmon microscopy

The localized properties of surface plasmons (SPs) and surface waves can be measured with a modified confocal microscope. An interference signal arises from a locally generated reference close to normal incidence and the beam that forms the surface wave. A spatial light modulator can impose different phase shifts on the part of the incident light to recover the properties of the SP. We report a Hilbert transform method to recover the wavenumber with a single shot. The method is faster and potentially less expensive than previous approaches. The signal-to-noise ratio is equivalent to the phase-stepping method. The signal processing necessary to condition the signal is described.

Common-path surface plasmon interferometer with radial polarization

We present a common-path surface plasmon interferometer with radial polarization. We show how the V(z) effect, the output of the microscope versus defocus z, can be derived utilizing a radially polarized illumination and a virtual annulus. The measurement of the V(z) effect gives a strong signature of the surface plasmon propagation, which is functionally related to the material properties. We discuss the advantages of using radial polarization compared to linear polarization.

Surface plasmon microscopy by spatial light switching for label-free imaging with enhanced resolution

In this Letter, we describe spatially switched surface plasmon microscopy (ssSPM) based on two-channel momentum sampling. The performance evaluated with periodic nanowires in comparison with conventional SPM and bright-field microscopy shows that the resolution of ssSPM is enhanced by almost 15 times over conventional SPM. ssSPM provides an extremely simple way to attain diffraction limit in SPM and to go beyond for super-resolution in label-free microscopy techniques.

Time-lapse scanning surface plasmon microscopy of living adherent cells with a radially polarized beam

We report on a fibered high-resolution scanning surface plasmon microscope for long term imaging of living adherent cells. The coupling of a high numerical aperture objective lens and a fibered heterodyne interferometer enhances both the sensitivity and the long term stability of this microscope, allowing for time-lapse recording over several days. The diffraction limit is reached with a radially polarized illumination beam. Adherence and motility of living C2C12 myoblast cells are followed for 50 h, revealing that the dynamics of these cells change after 10 h. This plasmon enhanced evanescent wave microscopy is particularly suited for investigating cell adhesion, since it can not only be performed without staining of the sample but it can also capture in real time the exchange of extracellular matrix elements between the substrate and the cells.

Single-nanoparticle near-infrared surface plasmon resonance microscopy for real-time measurements of DNA hybridization adsorption

A novel 814 nm near-infrared surface plasmon resonance (SPR) microscope is used for the real-time detection of the sequence-selective hybridization adsorption of single DNA-functionalized gold nanoparticles. The objective-coupled, high numerical aperture SPR microscope is capable of imaging in situ the adsorption of single polystyrene and gold particles with diameters ranging from 450 to 20 nm onto a 90 μm × 70 μm area of a gold thin film with a time resolution of approximately 1-3 s. Initial real-time SPR imaging (SPRI) measurements were performed to detect the accumulation of 40 nm gold nanoparticles for 10 min onto a gold thin film functionalized with a 100% complementary DNA surface at concentrations from 5 pM to 100 fM by counting individual particle binding events. A 100% noncomplementary DNA surface exhibited virtually no nanoparticle adsorption. In contrast, in a second set of SPRI measurements, two component complementary/noncomplementary mixed DNA monolayers that contained a very small percentage of complementary sequences ranging from 0.1 to 0.001%, showed both permanent and transient hybridization adsorption of the gold nanoparticles that could be tracked both temporally and spatially with the SPR microscope. These experiments demonstrate that SPR imaging measurements of single biofunctionalized nanoparticles can be incorporated into bioaffinity biosensing methods at subpicomolar concentrations.

Guided wave microscopy: mastering the inverse problem

Surface plasmon microscopy is widely recognized for its high sensitivity to nanoscale dielectric or metallic structures confined in a close neighborhood of a gold surface. Recently, its coupling to high-numerical-aperture objective lenses pushed its resolution down to the diffraction limit. Here, we show that the same microscope configuration can be used to excite standing guided waves in asymmetric slabs, which definitely extends the range of applications of this type of microscopy from nano- to microscale structure imaging. We demonstrate experimentally on PPMA films that the V(Z) response of a scanning surface plasmon microscope can be Fourier inverted in order to obtain the reflectivity curve R(ν). When the guided waves are excited, R(ν) shows a finite number of sharp peaks corresponding to quantified guiding modes from which one can extract both the refractive index (RI) and the thickness of the layer at the point focused by the microscope. This device can thus be used to reconstruct RI and thickness contours of dielectric samples with a high spatial resolution.

Subwavelength plasmonics for graded-index optics on a chip

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

Wavelet-based decomposition of high resolution surface plasmon microscopy V(Z) curves at visible and near infrared wavelengths

Surface plasmon resonance is conventionally conducted in the visible range and, during the past decades, it has proved its efficiency in probing molecular scale interactions. Here we elaborate on the first implementation of a high resolution surface plasmon microscope that operates at near infrared (IR) wavelength for the specific purpose of living matter imaging. We analyze the characteristic angular and spatial frequencies of plasmon resonance in visible and near IR lights and how these combined quantities contribute to the V(Z) response of a scanning surface plasmon microscope (SSPM). Using a space-frequency wavelet decomposition, we show that the V(Z) response of the SSPM for red (632.8 nm) and near IR (1550 nm) lights includes the frequential response of plasmon resonance together with additional parasitic frequencies induced by the objective pupil. Because the objective lens pupil profile is often unknown, this space-frequency decomposition turns out to be very useful to decipher the characteristic frequencies of the experimental V(Z) curves. Comparing the visible and near IR light responses of the SSPM, we show that our objective lens, primarily designed for visible light microscopy, is still operating very efficiently in near IR light. Actually, despite their loss in resolution, the SSPM images obtained with near IR light remain contrasted for a wider range of defocus values from negative to positive Z values. We illustrate our theoretical modeling with a preliminary experimental application to blood cell imaging.

Surface plasmon microscopic sensing with beam profile modulation

Surface Plasmon microscopy enables measurement of local refractive index on a far finer scale than prism based systems. An interferometric or confocal system gives the so-called V(z) curve when the sample is scanned axially, which gives a measure of the surface plasmon propagation velocity. We show how a phase spatial light modulator (i) performs the necessary pupil function apodization (ii) imposes an angular varying phase shift that effectively changes sample defocus without any mechanical movement and (iii) changes the relative phase of the surface plasmon and reference beam to provide signal enhancement not possible with previous configurations.

Confocal surface plasmon microscopy with pupil function engineering

Surface Plasmon microscopy can measure local changes of refractive index on the micron scale. Interferometric plasmon imaging delivers quantitative high spatial resolution sensitive to refractive index. In addition the so called V(z) method allows image contrast to be controlled by varying the sample defocus without substantially degrading spatial resolution. Here, we show how a confocal system provides a simpler and more stable alternative. This system, however, places greater demands on the dynamic range of the system. We therefore use a spatial light modulator to engineer the microscope pupil function to suppress light that does not contribute to the signal.