Optofluidic microsystems with integrated vertical one-dimensional photonic crystals for chemical analysis

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nano machining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing that they are the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depth is discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Catalytic nickel silicide as an alternative to noble metals in metal-assisted chemical etching of silicon

Metal-assisted chemical etching (MACE) has received much attention from researchers because it can be used to fabricate plasma-free anisotropic etching profiles for semiconductors. However, the etching mechanism of MACE is based on the catalytic reaction of noble metals, which restricts its use in complementary metal oxide semiconductor (CMOS) processes. To obtain process compatibility, we developed catalytic Ni after alloying it with Si as a substitute for noble metals in the MACE of Si substrates. Nickel silicide is a material commonly used as a contact electrode in CMOS processes. When NiSi was used as the catalyst, the anisotropic etching of Si with a smooth surface was successfully demonstrated. Silicidation increased the standard reduction potential of the Ni alloy and enhanced the electrochemical stability in the MACE of Si. In contrast, when pure Ni was used as the catalyst, a rough-etched surface was fabricated because of the low standard reduction potential. Based on the experimental results, the factors affecting the MACE of Si were systematically analyzed to optimize the catalytic NiSi properties. The implementation of the NiSi alloy potentially eliminates the use of noble metals in MACE and allows the technology to be adopted in contemporary CMOS processes.

Microfluidic Chip with Fiber-Tip Sensors for Synchronously Monitoring Concentration and Temperature of Glucose Solutions

Monitoring the properties of fluids in microfluidic chips often requires complex open-space optics technology and expensive equipment. In this work, we introduce dual-parameter optical sensors with fiber tips into the microfluidic chip. Multiple sensors were distributed in each channel of the chip, which enabled the real-time monitoring of the concentration and temperature of the microfluidics. The temperature sensitivity and glucose concentration sensitivity could reach 314 pm/°C and -0.678 dB/(g/L), respectively. The hemispherical probe hardly affected the microfluidic flow field. The integrated technology combined the optical fiber sensor with the microfluidic chip and was low cost with high performance. Therefore, we believe that the proposed microfluidic chip integrated with the optical sensor is beneficial for drug discovery, pathological research and material science investigation. The integrated technology has great application potential for micro total analysis systems (μ-TAS).

Slab waveguide-based particle plasmon resonance optofluidic biosensor for rapid and label-free detection

We demonstrate a cost-effective, rapid, and sensitive slab waveguide-based particle plasmon resonance biosensor with enhanced optical near field through the localized surface plasmon resonance effect for practical clinical applications.

Rapid and Highly Sensitive Detection of C-Reaction Protein Using Robust Self-Compensated Guided-Mode Resonance BioSensing System for Point-of-Care Applications

The rapid and sensitive detection of human C-reactive protein (CRP) in a point-of-care (POC) may be conducive to the early diagnosis of various diseases. Biosensors have emerged as a new technology for rapid and accurate detection of CRP for POC applications. Here, we propose a rapid and highly stable guided-mode resonance (GMR) optofluidic biosensing system based on intensity detection with self-compensation, which substantially reduces the instability caused by environmental factors for a long detection time. In addition, a low-cost LED serving as the light source and a photodetector are used for intensity detection and real-time biosensing, and the system compactness facilitates POC applications. Self-compensation relies on a polarizing beam splitter to separate the transverse-magnetic-polarized light and transverse-electric-polarized light from the light source. The transverse-electric-polarized light is used as a background signal for compensating noise, while the transverse-magnetic-polarized light is used as the light source for the GMR biosensor. After compensation, noise is drastically reduced, and both the stability and performance of the system are enhanced over a long period. Refractive index experiments revealed a resolution improvement by 181% when using the proposed system with compensation. In addition, the system was successfully applied to CRP detection, and an outstanding limit of detection of 1.95 × 10-8 g/mL was achieved, validating the proposed measurement system for biochemical reaction detection. The proposed GMR biosensing sensing system can provide a low-cost, compact, rapid, sensitive, and highly stable solution for a variety of point-of-care applications.

Impact of Fabrication and Bioassay Surface Roughness on the Performance of Label-Free Resonant Biosensors Based On One-Dimensional Photonic Crystal Microcavities

Micro- and nanofabrication offer remarkable opportunities for the preparation of label-free biosensors exploiting optical resonances to improve sensitivity and reduce detection limit once specificity is imparted through surface biofunctionalization. Nonetheless, both surface roughness, peculiar of fabrication processes, and bioassay roughness, resulting from uneven molecular coverage of the sensing surfaces, produce light scattering and, in turn, deterioration of biosensing capabilities, especially in resonant cavities where light travels forth and back thousands to million times. Here, we present a quantitative theoretical analysis about the impact of fabrication and bioassay surface roughness on the performance of optical biosensors exploiting silicon-based, vertical one-dimensional (1D) photonic crystal resonant cavities, also taking noise sources into account. One-dimensional photonic crystal resonant cavities with different architectures and quality factors ranging from 10² to 10⁶ are considered. The analysis points out that whereas sensitivity and linearity of the biosensors are not affected by the roughness level, either due to fabrication or bioassay, the limit of detection can be significantly degraded by both of them, depending on the quality factor of the cavity and noise level of the measurement system. The paper provides important insights into performance versus design, fabrication, and readout of biosensors based on resonant 1D photonic crystal cavities for real-setting operation.

Low-cost planar waveguide-based optofluidic sensor for real-time refractive index sensing

We report on the design, fabrication, and characterization of mass-producible, sensitive, intensity-detection-based planar waveguide sensors for rapid refractive index (RI) sensing; the sensors comprise suspended glass planar waveguides on glass substrates, and are integrated with microfluidic channels. They are facilely and cost-effectively constructed via vacuum-less processes. They yield a high throughput, enabling mass production. The sensors respond to solutions with different RIs via variations in the transmitted optical power due to coupling loss in the sensing region, facilitating real-time and simple RI detection. Experiments yield a good resolution of 5.65 × 10^-4 RIU. This work has major implications for several RI-sensing-based applications.

In-situ time resolved spectrographic measurement using an additively manufactured metallic micro-fluidic analysis platform

Introduction

Microfluidic reactionware allows small volumes of reagents to be utilized for highly controlled flow chemistry applications. By integrating these microreactors with onboard analytical systems, the devices change from passive ones to active ones, increasing their functionality and usefulness. A pressing application for these active microreactors is the monitoring of reaction progress and intermediaries with respect to time, shedding light on important information about these real-time synthetic processes.

Objective

In this multi-disciplinary study the objective was to utilise advanced digital fabrication to research metallic, active microreactors with integrated fibre optics for reaction progress monitoring of solvent based liquids, incompatible with previously researched polymer devices, in combination with on-board Ultraviolet-visible spectroscopy for real-time reaction monitoring.

Method

A solid-state, metal-based additive manufactured system (Ultrasonic Additive Manufacturing) combined with focussed ion beam milling, that permitted the accurate embedment of delicate sensory elements directly at the point of need within aluminium layers, was researched as a method to create active, metallic, flow reactors with on-board sensing. This outcome was then used to characterise and correctly identify concentrations of UV-active water-soluble B-vitamin nicotinamide and fluorescein. A dilution series was formed from 0.01–1.75 mM; which was pumped through the research device and monitored using UV-vis spectroscopy.

Results

The results uniquely showed the in-situ ion milling of ultrasonically embedded optical fibres resulted in a metallic microfluidic reaction and monitoring device capable of measuring solvent solutions from 18 μM to 18 mM of nicotinamide and fluorescein, in real time. This level of accuracy highlights that the researched device and methods are capable of real-time spectrographic analysis of a range of chemical reactions outside of those possible with polymer devices.

Nouman W, Abd El-Ghany SS, M.Sallam S, B. Dawood AF. Theoretical studies on hemoglobin periodic structure sensor

In the present study, we have obtained a blood hemoglobin (Hb) sensor using binary defective one-dimensional photonic crystal. The structure is composed of Air/Diamond/SiO2)NHb /Diamond/SiO2)S/SiO2 and the defect layer is filled by hemoglobin solution. The numerical calculations are based on transfer matrix method (TMM). The defect peak showed well shifting of the defect peak frequency by increasing the hemoglobin concentration; the wavelength shifted due to the change of hemoglobin concentration; from 679.5 nm at the 0g/dL to 682.3 nm at 28.7 g/dL. The presented idea is very simple, and can potentially attract a wider audience when one considers the fact of constantly rising interest of the scientific community (especially biologists and physicians) in the diagnostic methods utilizing different types of the optical phenomena.  

High Anodic-Voltage Focusing of Charge Carriers in Silicon Enables the Etching of Regularly-Arranged Submicrometer Pores at High Density and High Aspect-Ratio

The anodic dissolution of silicon in acidic electrolytes is a well-known technology enabling the silicon machining to be accurately controlled down to the micrometer scale in low-doped n-type silicon electrodes. Attempts to scale down this technology to the submicrometer scale has shown to be challenging, though it premises to enable the fabrication of meso and nano structures/systems that would greatly impact the fields of biosensors and nanomedicine. In this work, we report on the electrochemical etching at high anodic voltages (up to 40 V) of two-dimensional regular arrays of millions pores per square centimeter (up to 30 × 10⁶ cm^-2) with sub-micrometric diameter (down to ~860 nm), high depth (up to ~40 μm), and high aspect-ratio (up to ~45) using low-doped n-type silicon electrodes (resistivity 3-8 Ω cm). The use of high anodic voltages, which are over one order of magnitude higher than that commonly used in electrochemical etching of silicon, tremendously improves hole focusing at the pore tips during the etching and enables, in turn, the control of electrochemical etching of submicrometer-sized pores when spatial period reduces below 2 μm. A theoretical model allows experimental results to be interpreted in terms of an electric-field-enhanced focusing of holes at the tip apex of the pores at high anodic voltages, with respect to the pore base, which leads to a smaller curvature radius of the tip apex and enables, in turn, the etching of pore tips to be preferentially sustained over time and space.

Near-Infrared Silicon Photonic Crystals with High-Order Photonic Bandgaps for High-Sensitivity Chemical Analysis of Water-Ethanol Mixtures

Aqueous solutions of alcohols are used in several applications, from pharmaceutics and biology, to chemical, biofuel, and food industries. Nonetheless, development of a simple, inexpensive, and portable sensing device for the quantification of water in water-ethanol mixtures remains a significant challenge. Photonic crystals (PhCs) operating at very high-order photonic bandgaps (PBGs) offer remarkable opportunities for the realization of chemical sensors with high sensitivity and low detection limit. However, high-order PhC structures have been mostly confined to mere theoretical speculations so far, their effective realization requiring microfabrication tools enabling the control of periodic refractive index modulations at the submicrometric scale with extremely high accuracy and precision. Here, we report both experimental and theoretical results on high-sensitivity chemical analysis using vertical, silicon/air 1D-PhCs with spatial period of 10 and 20 μm (namely, over 10 times the operation wavelength) featuring ultra-high-order PBGs in the near-infrared region (namely, up to 50th at 1.1 μm). Fabrication of high-order 1D-PhCs was carried out by electrochemical micromachining (ECM) of silicon, which allowed both surface roughness and deviation from vertical of etched structures to be controlled below 5 nm and 0.1%, respectively. Optical characterization of ECM-fabricated 1D-PhCs, which was performed by acquiring reflectivity spectra over the wavelength range 1-1.7 μm, highlighted the presence of ultra-high-order PBGs with minor optical losses (i.e., <1 dB in reflectivity) separated by deep reflectivity notches with high Q-factors (i.e., >6000), in good agreement with theoretical calculations. Remarkably, the use of high-order 1D-PhCs as refractometric transducers for the quantitative detection of traces of water in water-ethanol mixtures, allowed high sensitivity (namely, either 1000 nm/RIU or ∼0.4 nm/% of water), good detection limit (namely, 5 × 10^-3 RIU or ∼10% water), and excellent resolution (namely, either 6 × 10^-4 RIU or 1.6% of water) to be reliably achieved on a detection volume of about 168 fL.

Optofluidic refractive-index sensors employing bent waveguide structures for low-cost, rapid chemical and biomedical sensing

We propose and develop an intensity-detection-based refractive-index (RI) sensor for low-cost, rapid RI sensing. The sensor is composed of a polymer bent ridge waveguide (BRWG) structure on a low-cost glass substrate and is integrated with a microfluidic channel. Different-RI solutions flowing through the BRWG sensing region induce output optical power variations caused by optical bend losses, enabling simple and real-time RI detection. Additionally, the sensors are fabricated using rapid and cost-effective vacuum-less processes, attaining the low cost and high throughput required for mass production. A good RI solution of 5.31 10^-4 × RIU^-1 is achieved from the RI experiments. This study demonstrates mass-producible and compact RI sensors for rapid and sensitive chemical analysis and biomedical sensing.

Asymmetric nanofluidic grating detector for differential refractive index measurement and biosensing

Measuring small changes in refractive index can provide both sensitive and contactless information on molecule concentration or process conditions for a wide range of applications. However, refractive index measurements are easily perturbed by non-specific background signals, such as temperature changes or non-specific binding. Here, we present an optofluidic device for measuring refractive index with direct background subtraction within a single measurement. The device is comprised of two interdigitated arrays of nanofluidic channels designed to form an optical grating. Optical path differences between the two sets of channels can be measured directly via an intensity ratio within the diffraction pattern that forms when the grating is illuminated by a collimated laser beam. Our results show that no calibration or biasing is required if the unit cell of the grating is designed with an appropriate built-in asymmetry. In proof-of-concept experiments we attained a noise level equivalent to ∼10^-5 refractive index units (30 Hz sampling rate, 4 min measurement interval). Furthermore, we show that the accumulation of biomolecules on the surface of the nanochannels can be measured in real-time. Because of its simplicity and robustness, we expect that this inherently differential measurement concept will find many applications in ultra-low volume analytical systems, biosensors, and portable devices.

Flow-through micro-capillary refractive index sensor based on T/R spectral shift monitoring

We present a flow-through refractive index sensor for measuring the concentration of glucose solutions based on the application of rectangular glass micro-capillaries, with channel depth of 50 µm and 30 µm. A custom designed and 3D printed polymeric shell protects the tiny capillaries, ensuring an easier handling and interconnection with the macro-fluidic path. By illuminating the capillary with broadband radiation centered at λ~1.55 µm, both the transmitted (T) and reflected (R) optical spectrum from the capillary are detected with an optical spectrum analyzer, exploiting an all-fiber setup. Monitoring the spectral shift of the ratio T/R in response to increasing concentration of glucose solutions in water we have obtained sensitivities up to 530.9 nm/RIU and limit of detection in the range of 10^-5-10^-4 RIU. Experimental results are in agreement with the theoretically predicted principle of operation. After the demonstration of amplitude detection at a single wavelength, we finally discuss the impact of the capillary parameters on the sensitivity.

Optics-Integrated Microfluidic Platforms for Biomolecular Analyses

Compared with conventional optical methods, optics implemented on microfluidic chips provide small, and often much cheaper ways to interrogate biological systems from the level of single molecules up to small model organisms. The optical probing of single molecules has been used to investigate the mechanical properties of individual biological molecules; however, multiplexing of these measurements through microfluidics and nanofluidics confers many analytical advantages. Optics-integrated microfluidic systems can significantly simplify sample processing and allow a more user-friendly experience; alignments of on-chip optical components are predetermined during fabrication and many purely optical techniques are passively controlled. Furthermore, sample loss from complicated preparation and fluid transfer steps can be virtually eliminated, a particularly important attribute for biological molecules at very low concentrations. Excellent fluid handling and high surface area/volume ratios also contribute to faster detection times for low abundance molecules in small sample volumes. Although integration of optical systems with classical microfluidic analysis techniques has been limited, microfluidics offers a ready platform for interrogation of biophysical properties. By exploiting the ease with which fluids and particles can be precisely and dynamically controlled in microfluidic devices, optical sensors capable of unique imaging modes, single molecule manipulation, and detection of minute changes in concentration of an analyte are possible.

Ordered Silicon Pillar Arrays Prepared by Electrochemical Micromachining: Substrates for High-Efficiency Cell Transfection

Ordered arrays of silicon nano- to microscale pillars are used to enable biomolecular trafficking into primary human cells, consistently demonstrating high transfection efficiency can be achieved with broader and taller pillars than reported to date. Cell morphology on the pillar arrays is often strikingly elongated. Investigation of the cellular interaction with the pillar reveals that cells are suspended on pillar tips and do not interact with the substrate between the pillars. Although cells remain suspended on pillar tips, acute local deformation of the cell membrane was noted, allowing pillar tips to penetrate the cell interior, while retaining cell viability.

Refractometric micro-sensor using a mirrored capillary resonator

We report on a flow-through optical sensor consisting of a microcapillary with mirrored channels. Illuminating the structure from the side results in a complicated spectral interference pattern due to the different cavities formed between the inner and outer capillary walls. Using a Fourier transform technique to isolate the desired channel modes and measure their resonance shift, we obtain a refractometric detection limit of (6.3 ± 1.1) x 10^-6 RIU near a center wavelength of 600 nm. This simple device demonstrates experimental refractometric sensitivities up to (5.6 ± 0.2) x 10² nm/RIU in the visible spectrum, and it is calculated to reach 1540 nm/RIU with a detection limit of 2.3 x 10^-6 RIU at a wavelength of 1.55 µm. These values are comparable to or exceed some of the best Fabry-Perot sensors reported to date. Furthermore, the device can function as a gas or liquid sensor or even as a pressure sensor owing to its high refractometric sensitivity and simple operation.

Liquid Core ARROW Waveguides: A Promising Photonic Structure for Integrated Optofluidic Microsensors

In this paper, we introduce a liquid core antiresonant reflecting optical waveguide (ARROW) as a novel optofluidic device that can be used to create innovative and highly functional microsensors. Liquid core ARROWs, with their dual ability to guide the light and the fluids in the same microchannel, have shown great potential as an optofluidic tool for quantitative spectroscopic analysis. ARROWs feature a planar architecture and, hence, are particularly attractive for chip scale integrated system. Step by step, several improvements have been made in recent years towards the implementation of these waveguides in a complete on-chip system for highly-sensitive detection down to the single molecule level. We review applications of liquid ARROWs for fluids sensing and discuss recent results and trends in the developments and applications of liquid ARROW in biomedical and biochemical research. The results outlined show that the strong light matter interaction occurring in the optofluidic channel of an ARROW and the versatility offered by the fabrication methods makes these waveguides a very promising building block for optofluidic sensor development.

A Complete Optical Sensor System Based on a POF-SPR Platform and a Thermo-Stabilized Flow Cell for Biochemical Applications

An optical sensor platform based on surface plasmon resonance (SPR) in a plastic optical fiber (POF) integrated into a thermo-stabilized flow cell for biochemical sensing applications is proposed. This device has been realized and experimentally tested by using a classic receptor-analyte assay. For this purpose, the gold surface of the POF was chemically modified through the formation of a self-assembling monolayer. The surface robustness of the POF-SPR platform has been tested for the first time thanks to the flow cell. The experimental results show that the proposed device can be successfully used for label-free biochemical sensing. The final goal of this work is to achieve a complete, small-size, simple to use and low cost optical sensor system. The whole system with the flow cell and the optical sensor are extensively described, together with the experimental results obtained with an immunoglobulin G (IgG)/anti-IgG assay.

On the performance of label-free biosensors based on vertical one-dimensional photonic crystal resonant cavities

In this work three Fabry-Perot (FP) resonant cavities based on vertical silicon/air one-dimensional photonic crystals (1DPhCs) featuring different architectures and fluidic functionalities are designed, and the role of key design parameters on their ideal biosensing performance, i.e. surface sensitivity, limit of detection, range of linearity, is investigated. Numerical calculations of the transmission spectra of the 1DPhC FP resonant cavities using the Transfer Matrix Method (TMM), versus thickness of a biolayer simulating biomolecules (e.g. proteins) adsorbed on the 1DPhC FP cavity surfaces, show that biosensors with surface sensitivity up to 300 pm/nm, limit of detection down to 0.07 nm, and high linearity over the range 0-50 nm of biolayer thickness can be achieved.

Optofluidic approaches for enhanced microsensor performances

Optofluidics is a relatively young research field able to create a tight synergy between optics and micro/nano-fluidics. The high level of integration between fluidic and optical elements achievable by means of optofluidic approaches makes it possible to realize an innovative class of sensors, which have been demonstrated to have an improved sensitivity, adaptability and compactness. Many developments in this field have been made in the last years thanks to the availability of a new class of low cost materials and new technologies. This review describes the Italian state of art on optofluidic devices for sensing applications and offers a perspective for further future advances. We introduce the optofluidic concept and describe the advantages of merging photonic and fluidic elements, focusing on sensor developments for both environmental and biomedical monitoring.

Capillarity-driven (self-powered) one-dimensional photonic crystals for refractometry and (bio)sensing applications

The synergistic use of capillarity and photonic crystals for both refractometry and biosensing applications is demonstrated, from both theoretical and experimental points of view.

Label-free optical detection of cells grown in 3D silicon microstructures

We demonstrate high aspect-ratio photonic crystals that could serve as three-dimensional (3D) microincubators for cell culture and also provide label-free optical detection of the cells. The investigated microstructures, fabricated by electrochemical micromachining of standard silicon wafers, consist of periodic arrays of silicon walls separated by narrow deeply etched air-gaps (50 μm high and 5 μm wide) and feature the typical spectral properties of photonic crystals in the wavelength range 1.0-1.7 μm: their spectral reflectivity is characterized by wavelength regions where reflectivity is high (photonic bandgaps), separated by narrow wavelength regions where reflectivity is very low. In this work, we show that the presence of cells, grown inside the gaps, strongly affects light propagation across the photonic crystal and, therefore, its spectral reflectivity. Exploiting a label-free optical detection method, based on a fiberoptic setup, we are able to probe the extension of cells adherent to the vertical silicon walls with a non-invasive direct testing. In particular, the intensity ratio at two wavelengths is the experimental parameter that can be well correlated to the cell spreading on the silicon wall inside the gaps.