Optofluidic laser for dual-mode sensitive biomolecular detection with a large dynamic range

Dual optofluidic distributed feedback dye lasers for multiplexed biosensing applications

Integrated optofluidic devices have become subjects of high interest for rapid biosensor devices due to their unique ability to combine the fluidic processing of small volumes of microfluidics with the analysis capabilities of photonic structures. By integrating dynamically reconfigurable optofluidic lasers on-chip, complex coupling can be eliminated while further increasing the capabilities of sensors to detect an increasing number of target biomarkers. Here, we report a polydimethylsiloxane-based device with two on-chip fluidic distributed feedback (DFB) laser cavities that are integrated with an orthogonal analyte channel for multiplexed fluorescence excitation. One DFB grating is filled with 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dissolved in dimethyl sulfoxide. The second grating is filled with rhodamine 6G dissolved in a diluted ethylene glycol solution. We present characterization of both lasers through analysis of the lasing spectra for spectral narrowing along with a power series to observe threshold behavior. We then demonstrate simultaneous detection of two different fluorescent microbeads as a proof of concept for scalable, single biomarker analysis using on-chip optofluidic lasers.

Large dynamic range dual-mode pH sensors via dye-doped ionic liquid fiber optofluidic lasers

We report a fiber optofluidic laser (FOFL) using an RhB-doped ionic liquid (BmimPF6) as the gain medium and explore its application for large dynamic range highly sensitive pH sensing. Due to the high Q-factor of the FOFL and the unique merits of BmimPF6, lasing emission presents a threshold of only 0.61 μJ mm-1. Particularly, lasing emission behaviors are strongly dependent on the pH value of the gain medium, i.e., in the pH range 4.28-6.37, the lasing central wavelength blue-shifts monotonically with a sensitivity as high as 5.02 nm per pH unit, which we attribute to the conversion of the cationic form of RhB to the zwitterionic form caused by the deprotonation of the COOH group. Under alkaline conditions (pH 7.20-11.17), the lasing emission intensity exhibits a significant decrease and the corresponding lasing central wavelength is also blue-shifted due to the solvent effect. The sensitivity based on the wavelength shift is 3.03 nm per pH unit, which is 4-fold higher than that of fluorescence-based sensing, while the sensitivity based on the variation of the lasing emission intensity is almost three orders of magnitude higher than that of fluorescence-based sensing. Our work presents a novel dual sensing paradigm in response to different pH conditions, which can greatly improve the reliability and discrimination of pH sensing.

Highly sensitive, modification-free, and dynamic real-time stereo-optical immuno-sensor

Modification-free biosensing with high specificity and sensitivity is essential for miniaturized, online, integrated, and rapid, or even real-time molecular analyses. However, most optical biosensors are based on surface pre-modification or fluorescent labeling, and have either low sensitivity or low quality factor (Q). To address these difficulties, in this study, an optical sensor prototype was developed with a microbubble optofluidic channel integrated inside a Fabry-Pérot cavity to three-dimensionally tailor the intra-cavity light field via the intra-cavity lensing (microbubble) configuration. A high Q-factor (∼105), small mode volume, and high light energy density were experimentally achieved with this "stereo-sensor" while maintaining an ultrahigh refractive index (RI) sensitivity (679 nm/RIU) and ultra-small RI resolution (∼10-7 RIU at 950 nm). Moreover, specific detection of very low concentration of biomolecules (5 fg/mL for human IgG and 0.5 pg/mL for human serum albumin (HSA)) and wide range of protein concentrations (e.g., fg/mL-ng/mL for human IgG and pg/mL-ng/mL for HSA) without probe pre-modification were achieved owing to the RI change specifically associated with the probe-target binding and the corresponding bio-macromolecular conformation change. This modification-free stereosensing scenario is applicable to continuous, real-time, and multiplexed operations, thus showing potential for online, integrated, dynamic, biomolecular analyses in vitro or in vivo, such as the dynamic metabolic analysis of single cells or organoids and point-of-care tests.

DNA Tetrahedron-Based Valency Controlled Signal Probes for Tunable Protein Detection

Combined detection of multiple markers related to the same disease could improve the accuracy of disease diagnosis. However, the abundance levels of multiple markers of the same disease varied widely in real samples, making it difficult for the traditional detection method to meet the requirements of a wide detection range. Herein, three kinds of cardiac biomarkers, cardiac troponin I (cTnI), myoglobin (Myo), and C-reaction protein (CRP), which were from the pM level to the μM level in real samples, were selected as model targets. Valency-controlled signal probes based on DNA tetrahedron nanostructures (DTNs) and platinum nanoparticles (PtNPs) were constructed for tunable cardiac biomarker detection. PtNPs with high horseradish peroxidase-like activity and stability served as signal molecules, and DTNs with unique spatial structure and sequence specificity were used for precisely controlling the number of connected PtNPs. By controlling the number of PtNPs connected to DTNs, monovalent, bivalent, and trivalent signal probes were obtained and were used for the detection of cardiac markers in different concentration ranges. The limit of detection of cTnI, Myo, and CRP was 3.0 pM, 0.4 nM, and 6.7 nM, respectively. Furthermore, it performed satisfactorily for the detection of cardiac markers in 10% human serum. It was anticipated that the design of valency-controlled signal probes based on DTNs and nanozymes could be extended to the construction of other multi-target detection platforms, thus providing a basis for the development of a new precision medical detection platform.

Monitoring Various Bioactivities at the Molecular, Cellular, Tissue, and Organism Levels via Biological Lasers

The laser is considered one of the greatest inventions of the 20th century. Biolasers employ high signal-to-noise ratio lasing emission rather than regular fluorescence as the sensing signal, directional out-coupling of lasing and excellent biocompatibility. Meanwhile, biolasers can also be micro-sized or smaller lasers with embedded/integrated biological materials. This article presents the progress in biolasers, focusing on the work done over the past years, including the molecular, cellular, tissue, and organism levels. Furthermore, biolasers have been utilized and explored for broad applications in biosensing, labeling, tracking, bioimaging, and biomedical development due to a number of unique advantages. Finally, we provide the possible directions of biolasers and their applications in the future.

Optical Whispering-Gallery-Mode Microbubble Sensors

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

Aggregation-Induced Emission Luminogen-Based Dual-Mode Enzyme-Linked Immunosorbent Assay for Ultrasensitive Detection of Cancer Biomarkers in a Broad Concentration Range

The enzyme-linked immunosorbent assay (ELISA) is one of the most commonly used methods for measuring antibodies and antigens in biological samples. However, developing new ELISAs with high detection sensitivity and broad detection dynamic ranges without resorting to complicated signal processing and equipment setups remains a challenge. In this work, we report a strategy to simultaneously improve the detection sensitivity and broaden the dynamic range by replacing the chromogenic reagents used in traditional ELISAs with an aggregation-induced emission luminogen (AIEgen). The developed AIE-ELISA could generate complementary absorbance and fluorescence signals with a linear detection range of 1.6-25,000 pg/mL. The application of this dual-mode AIE-ELISA in the detection of the prostate-specific antigen (PSA) realized a limit of detection of 1.3 pg/mL (3.78 × 10^-14 M) and dynamic range improvement of approximately 2 orders of magnitude compared to a single-mode ELISA, which enabled it to discriminate a minor PSA difference in a patient's serum. The simpler experimental operation, faster enzyme response speed, and better photostability of AIEgen than the traditional chromogenic reagents used in ELISAs showed that our developed AIE-ELISA holds great potential in the fields of immunoassay, immunohistochemistry, and immunocytochemistry.

Tunable optofluidic microbubble lens

Optofluidic microlenses are one of the crucial components in many miniature lab-on-chip systems. However, many optofluidic microlenses are fabricated through complex micromachining and tuned by high-precision actuators. We propose a kind of tunable optofluidic microbubble lens that is made by the fuse-and-blow method with a fiber fusion splicer. The optical focusing properties of the microlens can be tuned by changing the refractive index of the liquid inside. The focal spot size is 2.8 µm and the focal length is 13.7 µm, which are better than those of other tunable optofluidic microlenses. The imaging capability of the optofluidic microbubble lens is demonstrated under a resolution test target and the imaging resolution can reach 1 µm. The results indicate that the optofluidic microbubble lens possesses good focusing properties and imaging capability for many applications, such as cell counting, optical trapping, spatial light coupling, beam shaping and imaging.

Topological Encoded Vector Beams for Monitoring Amyloid-Lipid Interactions in Microcavity

Lasers are the pillars of modern photonics and sensing. Recent advances in microlasers have demonstrated its extraordinary lasing characteristics suitable for biosensing. However, most lasers utilized lasing spectrum as a detection signal, which can hardly detect or characterize nanoscale structural changes in microcavity. Here the concept of amplified structured light-molecule interactions is introduced to monitor tiny bio-structural changes in a microcavity. Biomimetic liquid crystal droplets with self-assembled lipid monolayers are sandwiched in a Fabry-Pérot cavity, where subtle protein-lipid membrane interactions trigger the topological transformation of output vector beams. By exploiting Amyloid β (Aβ)-lipid membrane interactions as a proof-of-concept, it is demonstrated that vector laser beams can be viewed as a topology of complex laser modes and polarization states. The concept of topological-encoded laser barcodes is therefore developed to reveal dynamic changes of laser modes and Aβ-lipid interactions with different Aβ assembly structures. The findings demonstrate that the topology of vector beams represents significant features of intracavity nano-structural dynamics resulted from structured light-molecule interactions.

A sequentially bioconjugated optofluidic laser for wash-out-free and rapid biomolecular detection

Microstructures can improve both sensitivity and assay time in heterogeneous assays (such as ELISA) for biochemical analysis; however, it remains a challenge to perform the essential wash process in those microstructure-based heterogeneous assays. Here, we propose a sequential bioconjugation protocol to solve this problem and demonstrate a new type of fiber optofluidic laser for biosensing. Except for acting as an optical microresonator and a microstructured substrate, the miniaturized hollow optical fiber (HOF) is used as a microfluidic channel for storing and transferring reagents thanks to its capability in length extension. Through the capillary action, different reagents were sequentially withdrawn into the fiber for specific binding and washing purposes. By using the sequentially bioconjugated FOFL, avidin molecules are detected based on competitive binding with a limit of detection of 9.5 pM, ranging from 10 pM to 100 nM. It is demonstrated that a short incubation time of 10 min is good enough to allow the biomolecules to conjugate on the inner surface of the HOF. Owing to its miniaturized size, only 589 nL of liquid is required for incubation, which reduces the sample consumption and cost for each test. This work provides a tool to exploit the potential of microstructured optical fibers in high-performance biosensing.

Imaging-Based Optofluidic Biolaser Array Encapsulated with Dynamic Living Organisms

Optofluidic biolasers have emerged as promising tools for biomedical analysis due to their strong light-matter interactions and miniaturized size. Recent developments in optofluidic lasers have opened a new Frontier in monitoring biological processes. However, most biolasers require precise recording of the lasing spectrum at the single cavity level, which limits its application in high-throughput applications. Herein, a microdroplet laser array encapsulated with living Escherichia coli was printed on highly reflective mirrors, where laser emission images were employed to reflect the dynamic changes in living organisms. The concept of image-based lasing analysis was proposed by quantifying the integrated pixel intensity of the lasing image from whispering-gallery modes. Finally, dynamic interactions between E. coli and antibiotic drugs were compared under fluorescence and laser emission images. The amplification that occurred during laser generation enabled the quantification of tiny biological changes in the gain medium. Laser imaging presented a significant increase in integrated pixel intensity by 2 orders of magnitude. Our findings demonstrate that image-based lasing analysis is more sensitive to dynamic changes than fluorescence analysis, paving the way for high-throughput on-chip laser analysis of living organisms.

Single-resonator, stable dual-longitudinal-mode optofluidic microcavity laser based on a hollow-core microstructured optical fiber

A single-resonator, stable dual-longitudinal-mode optofluidic microcavity laser based on a hollow-core microstructured optical fiber is proposed and experimentally demonstrated. The resonator and microfluidic channel are integrated in the hollow-core region of the fiber, inside which a hexagonal silica ring is used as the only resonator of the laser. Experimental results show that with mixing a small amount of Rhodamine B into a 1 mM Rhodamine 6G solution to form a dual-dye solution as a gain medium, the laser obtained by the method of lateral pumping can operate at dual longitudinal modes, with a threshold of 90 nJ/mm². By adjusting the concentration of Rhodamine B, the lasing wavelength of the laser and the power ratio of the two wavelengths can be controlled. And because the laser emission is co-excited by different kinds of dye molecules, the mode competition is diminished, enabling the simultaneously efficient optical gain and therefore lasing at dual longitudinal modes stably with a maximum lasing intensity fluctuation of 3.2% within 30 minutes even if the dual longitudinal modes have the same linear polarization states. This work can open up promising opportunities for diverse applications in biosensing and medical diagnosis with high sensitivity and integrated photonics with compact structure.

In-fiber optofluidic online SERS detection of trace uremia toxin

In this Letter, we propose a microstructured in-fiber optofluidic surface-enhanced Raman spectroscopy (SERS) sensor for the initial inspection of uremia through the detection of unlabeled urea and creatinine. As a natural microfluidic device, microstructured hollow fiber has a special structure inside. Through chemical bonds, the SERS substrate can be modified and grown on the surface of the suspended core. Here, the silver nanoparticles (Ag NPs) are embedded on the poly diallyl dimethyl ammonium chloride-modified graphene oxide sheet to achieve the self-assembled SERS substrate. The reduced distance between Ag NPs can increase the strong hot spots that generate enhanced Raman signals. Therefore, it can effectively detect the Raman signal of unlabeled trace uremic toxin analytes (urea, creatinine) inside the optical fiber. The results show that under simulated biophysical conditions, the limit detection (LOD) for urea is 10-4M and the linearity is good, especially at the clinical conventional concentration range (2.5-6.5×10-3M). In addition, the online Raman detection of creatinine aqueous solution LOD is 10-6M, which also has good linearity. Significantly, this Letter provides a microstructured optofluidic in-fiber Raman sensor for the preliminary detection of uremia, which will have good development prospects in the field of clinical biomedicine.

Role of gold nanoparticles in advanced biomedical applications

Gold nanoparticles (GNPs) have generated keen interest among researchers in recent years due to their excellent physicochemical properties. In general, GNPs are biocompatible, amenable to desired functionalization, non-corroding, and exhibit size and shape dependent optical and electronic properties. These excellent properties of GNPs exhibit their tremendous potential for use in diverse biomedical applications. Herein, we have evaluated the recent advancements of GNPs to highlight their exceptional potential in the biomedical field. Special focus has been given to emerging biomedical applications including bio-imaging, site specific drug/gene delivery, nano-sensing, diagnostics, photon induced therapeutics, and theranostics. We have also elaborated on the basics, presented a historical preview, and discussed the synthesis strategies, functionalization methods, stabilization techniques, and key properties of GNPs. Lastly, we have concluded this article with key findings and unaddressed challenges. Overall, this review is a complete package to understand the importance and achievements of GNPs in the biomedical field.

Optofluidic gradient refractive index resonators using liquid diffusion for tunable unidirectional emission

Resonators have been used in a wide range of fields, such as biochemical detection and microscale lasers. In recent years, optofluidic resonators have attracted a significant amount of attention owing to their unique liquid environments. Liquids containing biochemical samples can be designed to pass through the ring resonators or to directly form droplets, for sample sensing. Liquid diffusion is an important property in optofluidic applications, such as gradient refractive index lenses and waveguides. However, liquid diffusion has not been used in the study of optofluidic resonators, for both possible sensing characteristics, and unidirectional emission that is mostly acted as light sources. Here, we introduce a gradient refractive index profile formed by liquid diffusion in annular channels into a circular resonator, forming a gradient-index resonator with a tunable unidirectional emission. For both simulations and experiments, the squeezed and non-rotationally symmetrical light intensity profile was first obtained in a circular resonator. The squeezed light profile enables unidirectional emission in circular resonators, which is difficult to achieve in conventional ones. The squeezed light profile and unidirectional emission are determined by the refractive index difference of the liquids used, the dimension of the circular channels, and the working wavelengths. In experiments, different dimensions of bending radii were demonstrated and a tunable squeezed light intensity profile and unidirectional emission were exhibited. Interestingly, the squeezed coefficient of light, which was about 1.8 for a bending radius of 100 μm, enabled emission with a divergence angle as small as 14 degrees, which could be used for laser emission applications in the future. This work reveals the significant potential of the novel liquid gradient refractive index resonator, which provides a practicable approach for optofluidic resonator emission applications and also has potential for use in optofluidic sensing based on the squeezed light profile.

Ultrasensitive optofluidic enzyme-linked immunosorbent assay by on-chip integrated polymer whispering-gallery-mode microlaser sensors

Optical whispering-gallery-mode (WGM) microcavities offer great promise in ultrasensitive biosensors because of their unique ability to enable resonant recirculation of light to achieve strong light-matter interactions in microscale volumes. However, it remains a challenge to develop cost-effective, high-performance WGM microcavity-based biosensing devices for practical disease diagnosis applications. In this paper, we present an optofluidic chip that is integrated with directly-printed, high-quality-factor (Q) polymer WGM microlaser sensors for ultrasensitive enzyme-linked immunosorbent assay (ELISA). Optical 3D μ-printing technology based on maskless ultraviolet lithography is developed to rapidly fabricate high-Q suspended-disk WGM microcavities. After deposition with a thin layer of optical gain material, low-threshold WGM microlasers are fabricated and then integrated together with optical fibres upon a microfluidic chip to achieve an optofluidic device. With flexible microfluidic technology, on-chip, integrated, WGM microlasers are further modified in situ with biomolecules on surface for highly selective biomarker detection. It is demonstrated that such an optofluidic biochip can measure horseradish peroxidase (HRP)-streptavidin, which is a widely used catalytic molecule in ELISA, via chromogenic reaction at the concentration level of 0.3 ng mL-1. Moreover, it enables on-chip optofluidic ELISA of the disease biomarker vascular endothelial growth factor (VEGF) at the extremely low concentration level of 17.8 fg mL-1, which is over 2 orders of magnitude better than the ability of current commercial ELISA kits.

Optofluidic microbubble Fabry-Pérot cavity

An optofluidic microbubble Fabry-Pérot (OMBFP) cavity was investigated. In contrast to plane-plane FP (PPFP) cavities, the optical mode confinement and stability in an OMBFP were significantly enhanced. The optical properties of the OMBFP cavity, including the quality (Q) factor, effective mode area, mode distribution as a function of the core refractive index, microbubble position, and mirror tilt angle, were investigated systematically using the finite element method. In optofluidic lasing experiments, a low lasing threshold of 1.25 µJ/mm², which was one order magnitude lower than that of the PPFP, was achieved owing to improved modal lateral confinement. Since the microbubble acts as a micro-lens and microfluidic channel in the parallel FP cavity, mode selection and cell-dye laser were easily realized in the OMBFP cavity.

Mass production of thin-walled hollow optical fibers enables disposable optofluidic laser immunosensors

Disposable biosensors are of great importance in disease diagnosis due to their inherent merits of no cross-contamination and ease of use. Optofluidic laser (OFL) sensors are a new category of sensitive biosensors; however, it is challenging to cost-effectively mass-produce them to achieve disposability. Here, we report a disposable optofluidic laser immunosensor based on thin-walled hollow optical fibers (HOFs). Using a fiber draw tower, the fabrication parameters, including drawing speed and gas flow rate, are explored, and the HOF geometry is precisely controlled, which allows identical laser microring resonators to be distributed along the fibers. The disposable OFL immunosensor detects the protein concentration in the HOF through a wash-free immunoassay. Enabled by the disposable sensors, the statistical characteristics of 80 tests for each concentration greatly reduces the bioassay uncertainty. A low coefficient of variation (CV) of 3.3% confirms the high reproducibility of the disposable HOF-OFL sensors, and the mean of the normal distribution of the logarithmic OFL intensity serves as the sensing output. A limit of detection of 11 nM within a short assay time of 15 min is achieved. These disposable immunosensors possess the advantages of low cost, high reproducibility, fast assay, and low-volume consumption of sample and reagents. We believe that this work will inspire disposable optofluidics through the mass production of multifunctional microstructured optical fibers.

Fast and Reproducible ELISA Laser Platform for Ultrasensitive Protein Quantification

Optofluidic lasers are currently of high interest for sensitive intracavity biochemical analysis. In comparison with conventional methods such as fluorescence and colorimetric detection, optofluidic lasers provide a method for amplifying small concentration differences in the gain medium, thus achieving high sensitivity. Here, we report the development of an on-chip ELISA (enzyme-linked immunosorbent assay) laser platform that is able to complete an assay in a short amount of time with small sample/reagent volumes, large dynamic range, and high sensitivity. The arrayed microscale reaction wells in the ELISA lasers can be microfabricated directly on dielectric mirrors, thus significantly improving the quality of the reaction wells and detection reproducibility. The details of the fabrication and characterization of those reaction wells on the mirror are described and the ELISA laser assay protocols are developed. Finally, we applied the ELISA laser to detecting IL-6, showing that a detection limit of about 0.1 pg/mL can be achieved in 1.5 h with 15 μL of sample/reagents per well. This work pushes the ELISA laser a step closer to solving problems in real-world biochemical analysis.

Recent advances in nanomaterial-enhanced enzyme-linked immunosorbent assays

This review highlights functional roles of nanomaterials for advancing conventional ELISA assays by serving as substrate-alternatives, enzyme-alternatives, or non-enzyme amplifiers.

Spectral Modulation of Optofluidic Coupled-Microdisk Lasers in Aqueous Media

We present the spectral modulation of an optofluidic microdisk device and investigate the mechanism and characteristics of the microdisk laser in aqueous media. The optofluidic microdisk device combines a solid-state dye-doped polymer microdisk with a microfluidic channel device, whose optical field can interact with the aqueous media. Interesting phenomena, such as mode splitting and single-mode lasing in the laser spectrum, can be observed in two coupled microdisks under the pump laser. We modulated the spectra by changing the gap of the two coupled microdisks, the refractive indices of the aqueous media, and the position of a pump light, namely, selective pumping schemes. This optofluidic microlaser provides a method to modulate the laser spectra precisely and flexibly, which will help to further understand spectral properties of coupled microcavity laser systems and develop potential applications in photobiology and photomedicine.

Chromatin laser imaging reveals abnormal nuclear changes for early cancer detection

We developed and applied rapid scanning laser-emission microscopy (LEM) to detect abnormal changes in cell nuclei for early diagnosis of cancer and cancer precursors. Regulation of chromatins is essential for genetic development and normal cell functions, while abnormal nuclear changes may lead to many diseases, in particular, cancer. The capability to detect abnormal changes in "apparently normal" tissues at a stage earlier than tumor development is critical for cancer prevention. Here we report using LEM to analyze colonic tissues from mice at-risk for colon cancer (induced by a high-fat diet) by detecting pre-polyp nuclear abnormality. By imaging the lasing emissions from chromatins, we discovered that, despite the absence of observable lesions, polyps, or tumors under stereoscope, high-fat mice exhibited significantly lower lasing thresholds than low-fat mice. The low lasing threshold is, in fact, very similar to that of adenomas and is caused by abnormal cell proliferation and chromatin deregulation that can potentially lead to cancer. Our findings suggest that conventional detection methods, such as colonoscopy followed by histopathology, by itself, may be insufficient to reveal hidden or early tumors under development. We envision that this innovative work will provide new insights into LEM and support existing tools for early tumor detection in clinical diagnosis, and fundamental biological and biomedical research of chromatin changes at the biomolecular level of cancer development.

An infrared IgG immunoassay based on the use of a nanocomposite consisting of silica coated Fe3O4 superparticles

A reliable, rapid and ultrasensitive immunoassay is described for determination of immunoglobulin G (IgG). It is making use of biofunctional magnetite (Fe₃O₄) superparticles coated with SiO₂ and serving as an infrared (IR) probe. The unique IR fingerprint signals originating from the transverse and longitudinal phonon modes, respectively, of the asymmetric stretching of the Si-O-Si bridges display a satisfactory resistance to optical interference from the environment. The adoption of Fe₃O₄ superparticles instead of Fe₃O₄ nanoparticles as the magnetic core warrants a controllable structure and a strong magnetic response. This facilitates the efficient purification of the probes and the alleviation of the interfacial resistance between the liquid-solid interfaces by using a magnet. The gold-coated substrate was used to immobilize goat-anti-human IgG. The analyte (human IgG) was incubated with the IR probes, and then captured by the substrate immobilized antibody with the assistance of an external magnetic field. The integral area of the IR absorption band between 1250 cm^-1 - 900 cm^-1 was chosen for quantitative assay. The limit of detection is 95 fM, which is two orders of magnitude better than that without the magnetic field. The assay time was shortened from 2 h to 1 min. High selectivity, specificity, and long-term stability of the immunoassay were achieved. The performance of the assay when analyzing blood samples confirmed the practicability of the method. Graphical abstract Schematic presentation of the infrared (IR) immunoassay based on Fe₃O₄ superparticle@SiO₂ nanocomposites. The assistance of an external magnetic field reduces the incubation time and improves the detection sensitivity.

Regulatory-sequence mechanical biosensor: A versatile platform for investigation of G-quadruplex/label-free protein interactions and tunable protein detection

Mechanical biosensors can be used to quantitatively explore DNA-protein binding mechanisms by detecting targets at low concentrations or measuring force in single-molecule force spectroscopy. However, restrictions in single-molecule manipulation and labelling protocols have hindered the application for bulk analysis of label-free protein detection. Here, we present the integration of molecular force measurement and finely tunable detection of label-free proteins into one mechanical sensor. Regulatory-sequence force spectroscopy was obtained to investigate the binding force of DNA G-quadruplexes (GQ) and label-free protein. The dual control of regulatory sequences and mechanical forces induces the structure switching from DNA duplex to GQ/protein complex. It exhibits a synergistic effect, enabling the rational fine-tuning of the dynamic range for biosensing protein concentrations over eight orders of magnitude. Furthermore, this method was exploited to estimate the stability of the human telomeric DNA GQ by Ku protein and ligand methylpyridostatin. The results revealed that human telomeric GQ has two different binding sites for Ku protein and ligand. Force spectroscopy integrating label-free force measurement and tunable target detection holds great promise for use in biosensing, drug screening, targeted therapies, DNA nanotechnology, and fields in which GQ are of rapidly increasing importance.

Distributed fibre optofluidic laser for chip-scale arrayed biochemical sensing

Optofluidic lasers (OFLs) are an emerging technological platform for biochemical sensing, and their good performance especially high sensitivity has been demonstrated. However, high-throughput detection with an OFL remains a major challenge due to the lack of reproducible optical microcavities. Here, we introduce the concept of a distributed fibre optofluidic laser (DFOFL) and demonstrate its potential for high-throughput sensing applications. Due to the precise fibre geometry control via fibre drawing, a series of identical optical microcavities uniformly distributed along a hollow optical fibre (HOF) can be achieved to obtain a one-dimensional (1D) DFOFL. An enzymatic reaction catalysed by horseradish peroxidase (HRP) can be monitored over time, and the HRP concentration is detected by DFOFL-based arrayed colorimetric detection. Experimentally, five-channel detection in parallel with imaging has been demonstrated. Theoretically, spatial multiplexing of hundreds of channels is achievable with DFOFL-based detection. The DFOFL wavelength is tuned over hundreds of nanometers by optimizing the dye concentration or reconfiguring the liquid gain materials. Extending this concept to a two-dimensional (2D) chip through wavelength multiplexing can further enhance its multi-functionality, including multi-sample detection and spectral analysis. This work opens the door to high-throughput biochemical sensing.

A robust tissue laser platform for analysis of formalin-fixed paraffin-embedded biopsies

Laser emission-based detection and imaging technology has attracted significant interest in biomedical research due to its high sensitivity, narrow linewidth, and superior spectral and spatial resolution. Recent advances have further revealed the potential to use laser emission to investigate chromatin dynamics, as well as to diagnose cancer tissues based on nuclear biomarkers. To move the laser emission based detection technology a step further towards practical use, in this work, we developed a highly robust tissue laser platform by microfabricating an SU8 spacer with a fixed height on the top mirror of the Fabry-Pérot (FP) cavity, which allows generation of reproducible and stable lasing results regardless of tissue thickness. Then we applied this platform to achieve lasing emission from formalin-fixed, paraffin-embedded (FFPE) lung tissues, which account for an overwhelming fraction of tissues collected for research and clinical use worldwide. We further showed that the cancer and normal FFPE lung tissues can be distinguished by their respective lasing thresholds. Two different tissue thicknesses (10 μm and 5 μm) commonly used in pathological labs were explored. Finally, we tested three additional types of tissues (colon, stomach, and breast) that were prepared independently by lab technicians in a pathology lab in China and shipped to the US in order to validate the general applicability and practicality of the laser emission-based technology as well as the corresponding sample preparation protocol and the tissue laser platform. Our work will not only vastly broaden the applications of laser emission-based detection/imaging technology but also help translate it from the laboratory to an automated system for clinical practice that may eventually benefit biomedicine and biological research.

Advances of Optofluidic Microcavities for Microlasers and Biosensors

Optofluidic microcavities with high Q factor have made rapid progress in recent years by using various micro-structures. On one hand, they are applied to microfluidic lasers with low excitation thresholds. On the other hand, they inspire the innovation of new biosensing devices with excellent performance. In this article, the recent advances in the microlaser research and the biochemical sensing field will be reviewed. The former will be categorized based on the structures of optical resonant cavities such as the Fabry⁻Pérot cavity and whispering gallery mode, and the latter will be classified based on the working principles into active sensors and passive sensors. Moreover, the difficulty of single-chip integration and recent endeavors will be briefly discussed.

Reproducible fiber optofluidic laser for disposable and array applications

Disposable sensors are widely used in biomedical detection due to their inherent safety, ease of use and low cost. An optofluidic laser is a sensitive bioassay platform; however, demonstrating its fabrication cheaply and reproducibly enough for disposable use has been challenging. Here, we report a low-cost, reproducible fiber optofluidic laser (FOFL) using a microstructured optical fiber (MOF). The MOF not only supports the whispering gallery modes for lasing but also serves as a microfluidic channel for sampling the liquid gain medium via capillary force. Because of the precise control of its geometry (δ < 0.4%) during the fiber-drawing process, good reproducibility in laser intensity (δ = 6.5%) was demonstrated by changing 10 sections of the MOF. The strong coupling between the in-fiber resonator and gain medium enables a low threshold of 3.2 μJ mm^-2. The angular dependence of the laser emission was observed experimentally and analyzed with numerical simulations. An array of the FOFLs was also demonstrated. This technology has great potential for low-cost bioassay applications.

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.

Versatile tissue lasers based on high-Q Fabry-Pérot microcavities

Biolasers are an emerging technology for next generation biochemical detection and clinical applications. Progress has recently been made to achieve lasing from biomolecules and single living cells. Tissues, which consist of cells embedded in an extracellular matrix, mimic more closely the actual complex biological environment in a living body and therefore are of more practical significance. Here, we developed a highly versatile tissue laser platform, in which tissues stained with fluorophores are sandwiched in a high-Q Fabry-Pérot microcavity. Distinct lasing emissions from muscle and adipose tissues stained respectively with fluorescein isothiocyanate (FITC) and boron-dipyrromethene (BODIPY), and hybrid muscle/adipose tissue with dual staining were achieved with a threshold of only ∼10 μJ mm-2. Additionally, we investigated how the tissue structure/geometry, tissue thickness, and staining dye concentration affect the tissue laser. Lasing emission from FITC conjugates (FITC-phalloidin) that specifically target F-actin in muscle tissues was also realized. It is further found that, despite the large fluorescence spectral overlap between FITC and BODIPY in tissues, their lasing emissions could be clearly distinguished and controlled due to their narrow lasing bands and different lasing thresholds, thus enabling highly multiplexed detection. Our tissue laser platform can be broadly applicable to various types of tissues/diseases. It provides a new tool for a wide range of biological and biomedical applications, such as diagnostics/screening of tissues and identification/monitoring of biological transformations in tissue engineering.

Lasing in blood

Indocyanine green (ICG) is the only near-infrared dye approved by the U.S. Food and Drug Administration for clinical use. When injected in blood, ICG binds primarily to plasma proteins and lipoproteins, resulting in enhanced fluorescence. Recently, the optofluidic laser has emerged as a novel tool in bio-analysis. Laser emission has advantages over fluorescence in signal amplification, narrow linewidth, and strong intensity, leading to orders of magnitude increase in detection sensitivity and imaging contrast. Here we successfully demonstrate, to the best of our knowledge, the first ICG lasing in human serum and whole blood with the clinical ICG concentrations and the pump intensity far below the clinically permissible level. Furthermore, we systematically study ICG laser emission within each major serological component (albumins, globulins, and lipoproteins) and reveal the critical elements and conditions responsible for lasing. Our work marks a critical step toward eventual clinical and biomedical applications of optofluidic lasers using FDA approved fluorophores, which may complement or even supersede conventional fluorescence-based sensing and imaging.

Optofluidic chlorophyll lasers

Chlorophylls are essential for photosynthesis and also one of the most abundant pigments on earth. Using an optofluidic ring resonator of extremely high Q-factors (>10(7)), we investigated the unique characteristics and underlying mechanism of chlorophyll lasers. Chlorophyll lasers with dual lasing bands at 680 nm and 730 nm were observed for the first time in isolated chlorophyll a (Chla). Particularly, a laser at the 730 nm band was realized in 0.1 mM Chla with a lasing threshold of only 8 μJ mm(-2). Additionally, we observed lasing competition between the two lasing bands. The presence of laser emission at the 680 nm band can lead to quenching or significant reduction of laser emission at the 730 nm band, effectively increasing the lasing threshold for the 730 nm band. Further concentration-dependent studies, along with theoretical analysis, elucidated the mechanism that determines when and why the laser emission band appears at one of the two bands, or concomitantly at both bands. Finally, Chla was exploited as the donor in fluorescence resonance energy transfer to extend the laser emission to the near infrared regime with an unprecedented wavelength shift as large as 380 nm. Our work will open a door to the development of novel biocompatible and biodegradable chlorophyll-based lasers for various applications such as miniaturized tunable coherent light sources and in vitro/in vivo biosensing. It will also provide important insight into the chlorophyll fluorescence and photosynthesis processes inside plants.

Optofluidic FRET lasers using aqueous quantum dots as donors

An optofluidic FRET (fluorescence resonance energy transfer) laser is formed by putting FRET pairs inside a microcavity acting as a gain medium. This integration of an optofluidic laser and the FRET mechanism provides novel research frontiers, including sensitive biochemical analysis and novel photonic devices, such as on-chip coherent light sources and bio-tunable lasers. Here, we investigated an optofluidic FRET laser using quantum dots (QDs) as FRET donors. We achieved lasing from Cy5 as the acceptor in a QD-Cy5 pair upon excitation at 450 nm, where Cy5 has negligible absorption by itself. The threshold was approximately 14 μJ mm(-2). The demonstrated capability of QDs as donors in the FRET laser greatly improves the versatility of optofluidic laser operation due to the broad and large absorption cross section of the QDs in the blue and UV spectral regions. The excitation efficiency of the acceptor molecules through a FRET channel was also analyzed, showing that the energy transfer rate and the non-radiative Auger recombination rate of QDs play a significant role in FRET laser performance.

NaCl ion detection using a silica toroid microcavity

An optical-microcavity-based sensor is an important building block for an optofluidics system, because it allows us to fabricate small devices with high sensitivity. Here we describe the detection of NaCl and pH in water using a silica toroid microcavity. First we demonstrate the detection of NaCl particles, and show that a detection sensitivity of 0.38 mM is possible with a sample volume of 0.03 nl. Then, we report the detection of NaCl ions in liquid and demonstrate a sensitivity of 3.20 mM and also, in principle, the detection of pH with a sensitivity of 0.14 pH. Finally, we integrate a tapered optical fiber, a silica toroid microcavity, and a fluidic channel for future optofluidics applications.

Tunable optofluidic microring laser based on a tapered hollow core microstructured optical fiber

A tunable optofluidic microring dye laser within a tapered hollow core microstructured optical fiber was demonstrated. The fiber core was filled with a microfluidic gain medium plug and axially pumped by a nanosecond pulse laser at 532 nm. Strong radial emission and low-threshold lasing (16 nJ/pulse) were achieved. Lasing was achieved around the surface of the microfluidic plug. Laser emission was tuned by changing the liquid surface location along the tapered fiber. The possibility of developing a tunable laser within the tapered simplified hollow core microstructured optical fiber presents opportunities for developing liquid surface position sensors and biomedical analysis.

Nanostructured sensors for biomedical applications--a current perspective

Nanostructured sensors have unique capabilities that can be tailored to advantage in advancing the diagnosis, monitoring and cure of several diseases and health conditions. This report aims at providing a current perspective on, (a) the emerging clinical needs that defines the challenges to be addressed by nanostructured sensors, with specific emphasis on early stage diagnosis, drug-diagnostic combinations, and predictive models to design therapy, (b) the emerging industry trends in in vitro diagnostics, mobile health care, high-throughput molecular and cell-based diagnostic platforms, and (c) recent instances of nanostructured biosensors, including promising sensing concepts that can be enhanced using nanostructures that carry high promise towards catering to the emerging clinical needs, as well as the market/industry trends.

Optofluidic lasers with a single molecular layer of gain

We achieve optofluidic lasers with a single molecular layer of gain, in which green fluorescent protein, dye-labeled bovine serum albumin, and dye-labeled DNA, are used as the gain medium and attached to the surface of a ring resonator via surface immobilization biochemical methods. It is estimated that the surface density of the gain molecules is on the order of 10(12) cm(-2), sufficient for lasing under pulsed optical excitation. It is further shown that the optofluidic laser can be tuned by energy transfer mechanisms through biomolecular interactions. This work not only opens a door to novel photonic devices that can be controlled at the level of a single molecular layer but also provides a promising sensing platform to analyze biochemical processes at the solid-liquid interface.