Integrating Whispering Gallery Mode Refractive Index Sensing with Capillary Electrophoresis Separations Using Phase Sensitive Detection

Electric field-enhanced backscatter interferometry detection for capillary electrophoresis

Backscatter interferometry (BSI) is a refractive index (RI) detection method that is easily integrated with capillary electrophoresis (CE) and is capable of detecting species ranging from inorganic ions to proteins without additional labels or contrast agents. The BSI signal changes linearly with the square of the separation voltage which has been used to quantify sample injection, but has not been explored as a potential signal enhancement mechanism in CE. Here we develop a mathematical model that predicts a signal enhancement at high field strengths, where the BSI signal is dominated by the voltage dependent mechanism. This is confirmed in both simulation and experiment, which show that the analyte peak area grows linearly with separation voltage at high field strengths. This effect can be exploited by adjusting the background electrolyte (BGE) to increase the conductivity difference between the BGE and analyte zones, which is shown to improve BSI performance. We also show that this approach has utility in small bore capillaries where larger separation fields can be applied before excess Joule heating degrades the separation. Unlike other optical detection methods that generally degrade as the optical pathlength is reduced, the BSI signal-to-noise can improve in small bore capillaries as the larger separation fields enhance the signal.

Silicon Photonic Microring Resonator Arrays as a Universal Detector for Capillary Electrophoresis

Electrophoretic separations conventionally rely on chromogenic, fluorogenic, or redox properties for analyte detection that, in many instances, involve chemical modification of samples prior to analysis. For analytes natively lacking chemical signatures, refractive index-based measurements are appealing as a method to detect these molecules without pretreatment. Microring resonators are a type of whispering gallery mode sensor capable of detecting bulk changes in refractive index. Here, we demonstrate the use of silicon photonic microring resonator arrays as a postcolumn detector for capillary electrophoresis. In this approach, we establish the universal detection capabilities of microrings through calibration with analytes lacking unique spectral signatures. Separations of small molecule mixtures are demonstrated using capillary zone electrophoresis. For these separations, the microring resonators maintain a linear response over several orders of magnitude in concentration for three candidate small molecules. Successful separation of three sugars with direct detection is also demonstrated. We further present the successful separation and detection of three model proteins, exemplifying the promise of microring resonators arrays as a biocompatible detector for capillary electrophoresis. Additionally, the spatially offset, array-based nature of the sensing platform enables real-time analysis of analyte mobility and performance characterization-a combination that is not typically provided using single-point detectors.

A linear mass concentration detector for solvent gradient polymer separations

Silicon photonic microring resonators are an optical sensor utilized here as a detector for gradient elution liquid chromatography of polymers. Universal refractive index based detection and a linear mass concentration response is observed.

Silicon Photonic Microring Resonator Arrays for Mass Concentration Detection of Polymers in Isocratic Separations

Molecular weight distribution (MWD) is often the most informative analytical parameter in polymer analysis, with gel permeation chromatography (GPC) being the most common approach for determining the MWD for polymer samples. Many industrially relevant polymers lack chromogenic or fluorogenic signatures, precluding use of spectroscopy-based detection. Universal detectors, such as evaporative light scattering and charged aerosol detectors, are nonlinear, limiting quantitative polymer analysis. Differential refractive index (dRI) detectors show linear mass concentration sensitivity but are limited for some analyses given that they are incompatible with gradient-based separations, have a limited dynamic range, and require extended thermal equilibration times. In this study, we investigated the utility of silicon photonic microring resonator arrays as a quantitative mass concentration detector for industrial polymer analysis. Microring resonators have optical properties that are sensitive to changes in refractive index, offer an extended dynamic range, have a broad solvent compatibility, and have a linear mass concentration detection for a range of molecular weights. Linear mass concentration detection for microrings was demonstrated through a series of isocratic GPC separations using narrow MWD polystyrene (PS) standards. This detection technology was then utilized in conjunction with conventional GPC detectors to analyze a series of broad MWD PS standards, with results in good agreement with dRI and UV/visible. These results demonstrate the potential of the microring resonator platform as a detector for industrial polymer analysis.

Synthesis of Transparent Aqueous ZrO2 Nanodispersion with a Controllable Crystalline Phase without Modification for a High-Refractive-Index Nanocomposite Film

The controllable synthesis of metal oxide nanoparticles is of fundamental and technological interest. In this article, highly transparent aqueous nanodispersion of ZrO₂ with controllable crystalline phase, high concentration, and long-term stability was facilely prepared without any modification via the reaction of inexpensive inorganic zirconium salt and sodium hydroxide in water under an acid surrounding, combined with hydrothermal treatment. The as-prepared transparent nanodispersion had an average particle size of 7 nm, a high stability of 18 months, and a high solid content of 35 wt %. ZrO₂ nanocrystals could be readily dispersed in many solvents with high polarity including ethanol, dimethyl sulfoxide, acetic acid, ethylene glycol, and N, N-dimethylformamide, forming stable transparent nanodispersions. Furthermore, highly transparent polyvinyl alcohol/ZrO₂ nanocomposite films with high refractive index were successfully prepared with a simple solution mixing route. The refractive index could be tuned from 1.528 to 1.754 (@ 589 nm) by changing the mass fraction (0-80 wt %) of ZrO₂ in transparent nanocomposite films.

Wavelength Modulated Back-Scatter Interferometry for Universal, On-Column Refractive Index Detection in Picoliter Volumes

Wavelength-modulated back scatter interferometry (M-BSI) is shown to improve the detection metrics for refractive index (RI) sensing in microseparations. In M-BSI, the output of a tunable diode laser is focused into the detection zone of a separation channel as the excitation wavelength is rapidly modulated. This spatially modulates the observed interference pattern, which is measured in the backscattered direction. Phase-sensitive detection using a split photodiode detector aligned on one fringe of the interference pattern is used to monitor RI changes as analytes are separated. Using sucrose standards, we report a detection limit of 700 μg/L in a 75 μm i.d. capillary at the 3σ level, corresponding to a detection volume of 90 pL. To validate the approach for electrophoretic separations, Na⁺ and Li⁺ were separated and detected with M-BSI and indirect-UV absorbance on the same capillary. A 4 mg/L NaCl and LiCl mixture leads to comparable separation efficiencies in the two detection schemes, with better signal-to-noise in the M-BSI detection, but less baseline stability. The latter arises in part from Joule heating, which influences RI measurements through the thermo-optic properties of the run buffer. To reduce this effect, a 25 μm i.d. capillary combined with active temperature control was used to detect the separation of sucrose, glucose, and lactose with M-BSI. The lack of suitable UV chromophores makes these analytes challenging to detect directly in ultrasmall volumes. Using a 55 mM NaOH run buffer, M-BSI is shown to detect the separation of a mixture of 174 mg/L sucrose, 97 mg/L glucose, and 172 mg/L lactose in a 15 pL detection volume. The universal on-column detection in ultrasmall volumes adds new capabilities for microanalysis platforms, while potentially reducing the footprint and costs of these systems.

Scanning resonator microscopy integrating phase sensitive detection

Scanning resonator microscopy (SRM) is a scanning probe technique that uses a small, optical resonator attached to the end of a conventional atomic force microscopy cantilever to simultaneously measure optical and topography properties of sample surfaces. In SRM, whispering gallery mode (WGM) resonances excited in the attached optical resonator shift in response to changes in surface refractive index (RI), providing a mechanism for mapping RI with high spatial resolution. In our initial report, the SRM tip was excited with a fixed excitation wavelength during sample scanning, which limits the approach. An improved method based on a wavelength modulation coupled with phase sensitive detection is reported here. This results in real-time characterization of WGM spectral shifts while eliminating complications arising from measurements based solely on signal intensity. This improved approach, combined with a modified tip design enabling integration of smaller resonators, is shown to enhance signal-to-noise and lead to sub-100 nm spatial resolution in the SRM optical image. The improved capabilities are demonstrated through measurements on thin dielectric and polymer films.

Optical Microresonators for Sensing and Transduction: A Materials Perspective

Optical microresonators confine light to a particular microscale trajectory, are exquisitely sensitive to their microenvironment, and offer convenient readout of their optical properties. Taken together, this is an immensely attractive combination that makes optical microresonators highly effective as sensors and transducers. Meanwhile, advances in material science, fabrication techniques, and photonic sensing strategies endow optical microresonators with new functionalities, unique transduction mechanisms, and in some cases, unparalleled sensitivities. In this progress report, the operating principles of these sensors are reviewed, and different methods of signal transduction are evaluated. Examples are shown of how choice of materials must be suited to the analyte, and how innovations in fabrication and sensing are coupled together in a mutually reinforcing cycle. A tremendously broad range of capabilities of microresonator sensors is described, from electric and magnetic field sensing to mechanical sensing, from single-molecule detection to imaging and spectroscopy, from operation at high vacuum to in live cells. Emerging sensing capabilities are highlighted and put into context in the field. Future directions are imagined, where the diverse capabilities laid out are combined and advances in scalability and integration are implemented, leading to the creation of a sensor unparalleled in sensitivity and information content.

Label-Free Biological and Chemical Sensing Using Whispering Gallery Mode Optical Resonators: Past, Present, and Future

Sensitive and rapid label-free biological and chemical sensors are needed for a wide variety of applications including early disease diagnosis and prognosis, the monitoring of food and water quality, as well as the detection of bacteria and viruses for public health concerns and chemical threat sensing. Whispering gallery mode optical resonator based sensing is a rapidly developing field due to the high sensitivity and speed of these devices as well as their label-free nature. Here, we describe the history of whispering gallery mode optical resonator sensors, the principles behind detection, the latest developments in the fields of biological and chemical sensing, current challenges toward widespread adoption of these devices, and an outlook for the future. In addition, we evaluate the performance capabilities of these sensors across three key parameters: sensitivity, selectivity, and speed.

Ringing phenomenon in chaotic microcavity for high-speed ultra-sensitive sensing

The ringing phenomenon in whispering-gallery-mode (WGM) microcavities has demonstrated its great potential for highly-sensitive and high-speed sensing. However, traditional symmetric WGM microcavities have suffered from an extremely low coupling efficiency via free-space coupling because the emission of symmetric WGMs is non-directional. Here we report a new approach for high-speed ultra-sensitive sensing using the ringing phenomenon in a chaotic regime. By breaking the rotational symmetry of a WGM microcavity and introducing chaotic behaviors, we show that the ringing phenomenon in chaotic WGM microcavities extends over both the positive and the negative frequency detune, allowing the ringing phenomenon to interact with analytes over a much broader bandwidth with a reduced dead time. Because the coupling of the chaotic microcavity is directional, it produces a significantly higher signal output, which improves its sensitivity without the need of a fiber coupler.

Recent Advances in Microscale Western Blotting

Western blotting is a ubiquitous tool used extensively in the clinical and research settings to identify proteins and characterize their levels. It has rapidly become a mainstay in research laboratories due to its specificity, low cost, and ease of use. The specificity arises from the orthogonal processes used to identify proteins. Samples are first separated based on size and then probed with antibodies specific for the protein of interest. This confirmatory approach helps avoid pitfalls associated with antibody cross-reactivity and specificity issues. While the technique has evolved since its inception, the last decade has witnessed a paradigm shift in Western blotting technology. The introduction of capillary and microfluidic platforms has significantly decreased time and sample requirements while enabling high-throughput capabilities. These advances have enabled Western analysis down to the single cell level in highly parallel formats, opening vast new opportunities for studying cellular heterogeneity. Recent innovations in microscale Western blotting are surveyed, and the potential for enhancing detection using advances in label-free biosensing is briefly discussed.

A label-free infrared opto-fluidic method for real-time determination of flow rate and concentration with temperature cross-sensitivity compensation

The ability to accurately measure the flow rate, concentration, and temperature in real-time in micro total analysis systems (μTAS) is crucial when improving their practical sensing capabilities within extremely small volumes. Our label-free infrared (1500-1600 nm) opto-fluidic method, presented in this study, utilizes a cantilever-based flow meter integrated with two parallel optical fiber Fabry-Perot interferometers (FPIs). The first FPI serves as an ultra-sensitive flow meter and includes a Fiber Bragg Grating (FBG) tip for localized temperature sensing. The second FPI has a fabricated photopolymer micro-tip for highly sensitive refractive index (RI) determination. In this work, we performed 3-D simulation analysis to characterize cantilever deflection as well as temperature distribution and its effect on the RI. The experimental results from temperature cross-sensitivity analysis lead to real-time measurement resolutions of 5 nL min^-1, 1 × 10^-6 RIU and 0.05 °C, for the flow rate, refractive index, and temperature, respectively.