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Aqueous capsules made from polymers typically allow their encapsulated cargo (e.g., drugs, dyes, proteins) to slowly diffuse out into the solvent through the capsule shell. In many applications, there is a need for a ‘hermetic’ seal to protect the cargo from the solvent over extended periods of time. Ideally, this hermetic seal should also be capable of being broken on-demand to enable cargo release. We demonstrate a new design for capsules having the above combination of properties. The key is to create the capsule shell from wax materials (alkanes and fatty acids) with a defined melting temperature Tm. Capsules can be loaded with any desired material, including strong acids or bases, reactive or unstable reagents (such as H2O2), and biopolymer gels. When sealed capsules are placed in water, no leakage is observed for over six weeks. The capsules can also encapsulate volatile liquids and remain air-tight over at least three weeks. On the other hand, under mild heat (above Tm, e.g., to 45 °C), the shell melts, releasing the core contents into the surrounding solvent. This provides a convenient thermal on–off “switch” for delivering contents from the capsules. The utility of these capsules is shown by implementing a nitrate-detection assay using hazardous chemicals (including H2SO4) sealed in the capsules. These “smart” capsules thus constitute a modular, mass-producible platform that could be useful in diverse applications.

Sample filling and discretization within thermoplastic 2D microwell arrays is investigated toward the development of low cost disposable microfluidics for passive sample discretization. By using a high level of contact angle asymmetry between the filling channel and microwell surfaces, a significant increase in the range of well geometries that can be successfully filled is revealed. The performance of various array designs is characterized numerically and experimentally to assess the impact of contact angle asymmetry and device geometry on sample filling and discretization, resulting in guidelines to ensure robust microwell filling and sample isolation over a wide range of well dimensions. Using the developed design rules, reliable and bubble-free sample filling and discretization is achieved in designs with critical dimensions ranging from 20 μm to 800 μm. The resulting devices are demonstrated for discretized nucleic acid amplification by performing loop-mediated isothermal amplification for the detection of the mecA gene associated with methicillin-resistant Staphylococcus aureus.

Improving the portability of diagnostic medicine is crucial for alleviating global access-to-care deficiencies. This requires not only designing devices that are small and lightweight, but also autonomous and independent of electricity. Here, we present a strategy for conducting automated multistep diagnostic assays using chemically generated, passively regulated heat. Ligation and polymerization reagents for rolling circle amplification of nucleic acids are separated by meltable phase-change partitions, thus replacing precise manual reagent additions with automated partition melting. To actuate these barriers and individually initiate the various steps of the reaction, field ration heaters exothermically generate heat in a thermos, whereas fatty acids embedded in a carbonaceous matrix passively buffer the temperature around their melting points. Achieving multistage temperature profiles extend the capability of instrument-free diagnostic devices and improve the portability of reaction automation systems built around phase-change partitions.

As molecular diagnostics move away from polymerase chain reaction (PCR) in order to target point-of-care testing applications, loop-mediated isothermal amplification (LAMP) is gaining popularity due its rapid, sensitive and specific detection with simpler instrumentation. However, while Taqman PCR enables real-time quantitative readout and multiplexed gene detection in single samples, analogous methods in LAMP are not yet broadly developed. To date, the real-time detection methods applied to LAMP involve turbidimetry or measuring fluorescence of an intercalator; however, both of these methods are nonspecific to the target of interest and do not allow for multiple gene detection in a single sample. Probe-based methods have been developed to address the need for specific target detection and multiplexed, one-pot reactions, but most of these methods have strict assay conditions and require the design of loop primers, which is not always possible. DARQ LAMP is a probe-based method that offers the most promise for quantitative and real-time multiplexed detection, as it has a relatively simple design and can be used in either a four-primer or six-primer system. However, previous work has only shown the assay to function well in a narrow range of reaction conditions, which is restrictive given that various LAMP assays require a broad range of conditions. In this work we investigate the use of the newest-generation strand-displacing polymerase and demonstrate that it has higher tolerance to reaction conditions than previous polymerases. Using the results from these studies, we demonstrate a single-reaction triplex assay for the detection of methicillin-resistant S. aureus (MRSA), which would not be possible with any of the previously reported LAMP systems.

The seamless integration of reagents into microfluidic devices can serve to significantly reduce assay complexity and cost for disposable diagnostics. In this work, the integration of multiplexed reagents into thermoplastic 2D microwell arrays is demonstrated using a scalable pin spotting technique. Using a simple and low-cost narrow-bore capillary spotting pin, high resolution deposition of concentrated reagents within the arrays of enclosed nanoliter-scale wells is achieved. The pin spotting method is further employed to encapsulate the deposited reagents with a chemically modified wax layer that serves to prevent disruption of the dried assay components during sample introduction through a shared microchannel, while also enabling temperature-controlled release after sample filling is complete. This approach supports the arbitrary patterning and release of different reagents within individual wells without crosstalk for multiplexed analyses. The performance of the in-well spotting technique is characterized using on-chip rolling circle amplification to evaluate its potential for nucleic acid-based diagnostics.

Meltable barriers are an attractive means to achieve controlled delivery of reagents in a variety of settings, enabling assays to be performed through thermal automation instead of manual addition of reactants. However, mixing kinetics in such systems can be slow due to the lack of active flow or mechanical shaking. We demonstrate a new strategy for hands-free, thermally-automated agitation of biochemical reactions. Reagents for binary effervescent reactions are lyophilized then capped with a phase-change partition, eicosane. This barrier can be melted at moderate temperatures, at which point an aqueous solution dissolves the reactants, generating bubbles that mix the solution through convection. We explore reactions that generate bubbles of carbon dioxide and oxygen gasses, characterizing the induced mixing rate of two aqueous solutions with dissimilar densities. This strategy affords control over the initiation and duration of convective mixing, providing a tool for thermal automation of biochemical reactions with efficient reaction kinetics.

Improving the portability of diagnostic medicine is crucial to alleviating global access-to-care deficiencies. This requires not only designing devices that are small and lightweight but also autonomous and independent of electricity. Here, we present a strategy for conducting automated multi-step diagnostic assays using chemically generated, passively regulated heat. Ligation and polymerization reagents for Rolling Circle Amplification of nucleic acids are separated by melt-able phase-change partitions, thus replacing precise manual reagent additions with automated partition melting. To actuate these barriers and individually initiate the various steps of the reaction, field ration heaters exothermically generate heat in a thermos while fatty acids embedded in a carbonaceous matrix passively buffer the temperature around their melting points. Achieving multi-stage temperature profiles extends the capability of instrument-free diagnostic devices and improves the portability of reaction automation systems built around phase-change partitions.

Surface enhanced Raman spectroscopy (SERS) has the potential to enable point-of-care sensing across the spectrum of chemical and biological analytes. In diagnostic assays, SERS has been demonstrated to increase the multiplexing density while reducing the burden of fluorescence hardware. One particular application of interest is the use of SERS to provide a multiplexed optical read-out following polymerase chain reaction (PCR). To date, however, the reported PCR–SERS assays require endpoint mixing with a plasmonic nanoparticle solution for detection, thus adding manual steps and preventing real-time, quantitative PCR. In this work, we detail a real-time PCR–SERS thermoplastic microsystem that allows simultaneous nucleic acid amplification and product separation into a SERS-active silver colloid for real-time detection. Specifically, a laser cut thermoplastic fluidic chip has been devised to utilize a dialysis membrane capable of isolating a PCR reaction from the silver colloid. As the reaction progresses, a Raman-reporter-labeled DNA probe is degraded, liberating the reporter from probe DNA, allowing passage across the size-restricting dialysis membrane into the SERS-active colloid, where the accumulating reporter can be measured in real time. Here, we demonstrate that this system is capable of real-time and single-well multiplexed readout of a PCR reaction to simultaneously detect two biomarker genes for methicillin-resistant S. aureus (MRSA).

While surface enhanced Raman spectroscopy (SERS) may not compete with the standard central lab approaches for chemical and biological sensing, SERS may have the potential to provide unique capabilities for analytics away from the central lab. Raman spectrometers have evolved from benchtop systems to high-performing handheld instruments that are compatible with analysis of samples in the field. However, for SERS to truly succeed as a “point-of-sample” analytical technique, the SERS sensor must fit the needs of analysis in the field, including little or no sample preparation, minimal peripheral equipment, and ease of use. Traditional plasmonically-active rigid devices do not meet these requirements. Even microfluidic SERS devices generally are not compatible with point-of-sample analysis, as the "world-to-chip" interface presents challenges, and peripheral equipment is generally required. In this review we will discuss the advances in plasmonic substrates fabricated on porous membranes, leading to SERS sensors that can collect samples via swabbing or dipping, clean up samples through separation, concentrate analytes by lateral flow focusing, and avoid the need for peripheral equipment. In particular, we will focus on inkjet-fabricated devices, which may present the best opportunity for scale-up via roll-to-roll manufacturing. We will also discuss the directions that flexible SERS sensors are moving the field, such as simple fabrication techniques, new support materials, SERS swabs, and SERS-active tapes and films.

A need exists for near real-time therapeutic drug monitoring (TDM), in particular for antibiotics and antifungals in patient samples at the point-of-care. To truly fit the point-of-care need, techniques must be rapid and easy to use. Here we report a membrane system utilizing inkjet-fabricated surface enhanced Raman spectroscopy (SERS) sensors that allows sensitive and specific analysis despite the elimination of sophisticated chromatography equipment, expensive analytical instruments, and other systems relegated to the central lab. We utilize inkjet-fabricated paper SERS sensors as substrates for 5FC detection; the use of paper-based SERS substrates leverages the natural wicking ability and filtering properties of microporous membranes. We investigate the use of microporous membranes in the vertical flow assay to allow separation of the flucytosine from whole blood. The passive vertical flow assay serves as a valuable method for physical separation of target analytes from complex biological matrices. This work further establishes a platform for easy, sensitive, and specific TDM of 5FC from whole blood.

The fabrication of substrates for surface enhanced Raman spectroscopy (SERS) by printing plasmonic structures on paper has emerged as a potential low-cost replacement for conventional nanofabricated SERS devices. Not only are the paper devices low in cost to produce, the inherent flexibility and fluidic transport capabilities of paper provide advantages in sample collection and processing, as well as analyte concentration. In this study, we review the recent progress in paper or membrane SERS devices fabricated through inkjet printing and other deposition methods. We then report a new potential diagnostics application for paper SERS devices that leverages the fluidic transport and chromatographic capabilities of paper to enable SERS-based detection following PCR. The use of paper SERS creates the potential for a simple yet densely multiplexed PCR assay that is not possible with conventional fluorescence-based transduction.

Surface enhanced Raman spectroscopy (SERS) provides rapid and sensitive identification of small molecule analytes. Traditionally, fabrication of SERS devices is an expensive process that involves the use of micro- and nano-fabrication procedures. Further, acquisition of diverse sample types requires complex preparation procedures that limits SERS to lab-based applications. Recent innovations using plasmonic nanoparticles embedded in flexible paper substrates has allowed the expansion of SERS techniques to portable analytical procedures. Recently inkjet-printing has been identified as a low cost, rapid, and highly customizable method for producing paper based SERS sensors with robust performance. This chapter details the materials and procedures by which inkjet printed SERS sensors can be fabricated and applied to relevant applications. In particular, methods for utilizing the sensors for detection of antibiotics are presented.

In this paper, we present an optofluidic SERS microsystem suitable for on-site detection of multiplexed analytics in the field. We show simultaneous detection of three fungicides that are highly regulated in aquaculture. The optofluidic SERS microsystem shows improved portability since it does not require a bulky pump for sample loading. The sample is simply drawn into the microchannel using a pipette. Additionally, two fiber optic cables are inserted into the device for sample excitation and photon collection. The fiber optic cables, aligned into the detection zone, eliminate the need for microscope alignment required in traditional SERS detection. The detection zone of the device consists of a porous matrix of packed silica microspheres that traps silver nanoparticles and adsorbed analyte molecules. The sample is passively concentrated into the matrix as it is loaded by applying negative pressure from the outlet with a pipette. The concentrating matrix has been shown to amplify the SERS signal by up to four orders of magnitude as compared to SERS in an open microfluidic channel. We were able to detect as low as 5 ppm methyl parathion, 0.1 ppb malachite green, and 5 ppb thiram simultaneously.

Surface enhanced Raman spectroscopy (SERS) leverages the specificity of Raman scattering and the sensitivity provided by localized plasmonic effects for applications in chemical and biomolecular detection. However, nearly four decades after the first report of SERS, practical uses of the technique remain limited. Optofluidic SERS—the synergistic use of microfluidics to improve the performance of SERS—may finally lead to practical devices for chemical and biomolecular detection. In this review, we describe recent advances in optofluidic SERS microsystems that have been developed to improve the performance and applicability of SERS. These techniques include designs that improve the light–analyte interaction, that perform active or passive concentration of metal nanoparticles and/or analyte molecules, and that utilize microfluidic techniques to improve functionality. In addition, we present optofluidic SERS techniques that enable new applications that have not been possible before the advent of optofluidics. Finally, we project future advances in optofluidic SERS and present a vision for the disruptive technologies that will enable the translation of SERS from the research lab to practical uses.

We demonstrate refractive index measurement of liquids using two sensor system designs, both based on microring resonators. Evanescent sensors based on microrings utilize the resonating nature of the light to dramatically decrease the required size and sample consumption volume, which are requirements of lab-on-a-chip sensor systems. The first design, which utilizes an optical microsphere, exhibits a sensitivity of 30 nm/RIU and a resulting detection limit on the order of 10-7 RIU. The second approach is a novel design called a liquid core optical ring resonator (LCORR). This concept uses a quartz capillary as the fluidics and as the ring resonator and achieves a sensitivity of 16.1 nm/RIU. The detection limit of this system is around 5times10-6 RIU. Both of these systems have the potential to be incorporated with advanced microfluidic systems for lab-on-a-chip applications. In particular, the LCORR combines high sensitivity, performance stability, and microfluidic compatibility, making it an excellent choice for lab-on-a-chip development

As a bio/chemical sensing technique, surface enhanced Raman spectroscopy (SERS) offers sensitivity comparable to that of fluorescence detection while providing highly specific information about the analyte. Although single molecule identification with SERS was demonstrated nearly 20 years ago, today a need exists to develop practical solutions for point‐of‐sample and point‐of‐care SERS systems. Recently, we demonstrated the fabrication of SERS substrates by inkjet printing silver and gold nanostructures onto paper and other microporous membranes. Using these devices, we have been able to achieve detection limits comparable to conventional nanofabricated plasmonic substrates. Furthermore, we leverage the fluidic properties of paper to enhance the performance of the SERS devices while also enabling unprecedented ease of use. Here we report the use of inkjet‐fabricated paper SERS substrates as a detection device for biological macromolecules in an easy‐to‐use format with a low number of steps. The targeted biomarker is specifically detected with SERS through a single step competitive displacement, which dramatically reduces the number of steps as compared to conventional assays. Moreover, we further improve the usability of the assay by incorporating a paper SERS device with a fluidic cartridge format. The wicking nature of the paper sensor eliminates manual sample application steps and is much simpler than the world‐to‐chip interface of microfluidic devices. The introduction of this paper‐based SERS assay is a significant step towards highly sensitive, low‐cost, and, importantly, easy to use multiplexed biological assays.

Fluorescence microscopy offers a number of advantages for cell- and biomarker-based diagnostics with regards to ease of use and interpretation, sensitivity, and specificity. However, its use in low-resource settings is often hindered by the need for bulky microscopes with expensive excitation and filter setups. While many advances have been made towards utilizing smartphones as microscopes, there remains a reliance on complex attachments to facilitate fluorescence microscopy. Here, we report progress towards a filter-less fluorescent assay utilizing ultraviolet light, an unmodified smartphone, and pyrene-labeled aptamers. The pyrene monomer is excited at a wavelength of 350 nm and emits at approximately 390 nm; when two pyrene molecules are brought into close proximity, however, they form an excimer which emits at approximately 490 nm. We have engineered pyrene-conjugated DNA sequences such that the fluorophores, normally in monomeric configuration, are brought into proximity upon binding of the DNA to its target. The large Stokes shift between excitation and emission of the excimer allows us to detect such biorecognition events with an unfiltered smartphone camera, enabling the use of this assay in low-resource settings where portability and easeof- use are paramount.

In this work, we introduce a method for the soft-lithography-based fabrication of rigid microstructures and a new, simple bonding technique for use as a continuous-flow cell lysis device. While on-chip cell lysis techniques have been reported previously, these techniques generally require a long on-chip residence time, and thus cannot be performed in a rapid, continuous-flow manner. Microstructured microfluidic devices can perform mechanical lysis of cells, enabling continuous-flow lysis; however, rigid silicon-based devices require complex and expensive fabrication of each device, while polydimethylsiloxane (PMDS), the most common material used for soft lithography fabrication, is not rigid and expands under the pressures required, resulting in poor lysis performance. Here, we demonstrate the fabrication of microfluidic microstructures from off-stoichiometry thiol-ene (OSTE) polymer using soft-lithography replica molding combined with a post-assembly cure for easy bonding. With finite element simulations, we show that the rigid microstructures generate an energy dissipation rate of nearly 10(7), which is sufficient for continuous-flow cell lysis. Correspondingly, with the OSTE device we achieve lysis of highly deformable MDA-MB-231 breast cancer cells at a rate of 85%, while a comparable PDMS device leads to a lysis rate of only 40%.

Surface enhanced Raman spectroscopy (SERS) is a powerful spectroscopic technique capable of detecting trace amounts of chemicals and identifying them based on their unique vibrational characteristics. While there are many complex methods for fabricating SERS substrates, there has been a recent shift towards the development of simple, low cost fabrication methods that can be performed in most labs or even in the field. The potential of SERS for widespread use will likely be realized only with development of cheaper, simpler methods. In this Perspective article we briefly review several of the more popular methods for SERS substrate fabrication, discuss the characteristics of simple SERS substrates, and examine several methods for producing simple SERS substrates. We highlight potential applications and future directions for simple SERS substrates, focusing on highly SERS active three-dimensional nanostructures fabricated by inkjet and screen printing and galvanic displacement for portable SERS analysis – an area that we believe has exciting potential for future research and commercialization.

Surface enhanced Raman spectroscopy (SERS) is a highly sensitive sensing technique, offering sensitivity comparable to that of fluorescence while providing structure-dependent analyte information. In recent years, we have developed an innovative optofluidic SERS substrate by inkjet printing metal nanoparticles onto paper. By virtue of generating a SERS substrate on cellulose, we gain a flexible SERS sensing device, as well as the ability to harness the intrinsic wicking properties of paper to enable both separation and concentration of analytes. Here we demonstrate the application of paper-chromatographic separation to allow on-substrate separation, concentration and discrimination. By using inexpensive single-labeled DNA probes in a typical PCR amplification, we obtain a mixture containing whole probes (negative result) and probes which have been hydrolyzed by the Taq polymerase (positive result). Leveraging the solubility differences between the whole and hydrolyzed probes and the cellulose separation matrix, we are able to perform a multiplexed interrogation of the targets. Notably, this does not require the use of dual labeled DNA probes (expensive) or multiple excitation sources and filter sets needed for a multiplexed fluorescence measurement (expensive and bulky). All SERS measurements are performed using a portable spectrometer and diode laser; in combination with a portable low-power DNA amplification system, this technique has the potential to be used for rapid on-site multiplexed genetic detection, without requiring complex optical equipment.

In principle, surface enhanced Raman spectroscopy (SERS) is thought to provide unique identification of a target analyte, even in complex samples or in the presence of multiple analytes. In practice, however, this is not always true for real-world samples due to various forms of interference. In this report, we build upon our previous work on inkjet-printed SERS substrates by using paper and polymer membranes to integrate sample cleanup and analyte separation with SERS detection. Inkjet-printed paper SERS substrates provide a highly sensitive chemical detection platform of unprecedented cost and simplicity. In addition, paper inherently provides unique capabilities, such as capillary-actuated fluid transport and selective molecular retention. Utilizing these properties, we demonstrate two-dimensional chromatographic separation and SERS detection on inkjet-printed paper SERS substrates. Then, we leverage the separation properties of paper and polymer membranes for real applications that feature complex sample matrices, including the detection of down to 5 ppm melamine in infant formula, as well as the quantification of nanograms of heroin in samples contaminated with a highly fluorescent background. The results presented here demonstrate that inkjet-printed paper SERS devices not only provide advantages in terms of sensitivity and cost, but the paper provides inherently integrated sample cleanup capabilities that are not available in traditional SERS substrates and microfluidic SERS devices. These unique capabilities of paper SERS devices enable the identification of targeted analytes even in complex real-world samples.

Surface enhanced Raman spectroscopy (SERS) has the potential to be utilized for the detection of a broad range of chemicals in trace quantities. However, because of the cost and complexity of SERS devices, the technology has been unable to fill the needs of many practical applications, in particular the need for rapid, portable, on-site detection in the field. In this work, we review a new methodology for trace chemical detection using inkjet-printed SERS substrates on paper. The detection performance of the inkjet-printed SERS devices is demonstrated by detecting 1,2-Bis(4-pyridyl)ethylene (BPE) at a concentration as low as 1.8 ppb. We then illustrate the primary advantages of paper SERS substrates as compared to conventional SERS substrates. By leveraging lateral flow concentration, the detection limit of paper SERS substrates can be further improved. Two real-world applications are demonstrated. First, the inkjet-printed SERS substrates are used as “dipsticks” for detecting the fungicide malachite green in water. Then, the flexible paper-based SERS devices are used as swabs to collect and detect trace residues of the fungicide thiram from a surface. We predict that the combination of ultra-low-cost fabrication with the advantages of easy-to-use dipsticks and swabs and the option of lateral flow concentration position ink-jet printed SERS substrates as a technology which will enable the application of SERS in solving critical problems for chemical detection in the field.

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages nanoporous microfluidics to dramatically increase the SERS performance. A number of optofluidic approaches have been used to improve the detection limit of SERS in microfluidic channels, including active concentration of nanoparticles and/or analyte and passive concentration of nanoparticles. Previous reports have used a single nanofabricated fluidic channel to trap metal nanoparticles and adsorbed analytes. In this work, we utilize a significantly simpler fabrication approach by packing silica beads in a microfluidic channel to create a 3D nanofluidic concentration matrix. The device is fabricated using polydimethylsiloxane (PDMS) on glass using typical soft lithography methods. Due to the larger area of the nanoporous fluidic channel, this approach should be less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. Using this microfluidic device, we achieved a detection limit of 4 femtomoles of Rhodamine 6G in 2 minutes. Compared to an open microfluidic channel, the 3D nanoporous concentration matrix increased the SERS signal by a factor of 250 due to the trapping of silver nanoclusters. Fiber optic cables are integrated into the PDMS to deliver excitation light directly to the detection volume and to collect Raman-scattered photons. As a result, the use of a laser diode and alignment-free integrated fiber optics implies the potential for the device to be used in portable and automated applications, such as the on-site detection of pesticides, water contaminants, and explosives.

It is known that tumor-initiating cells with stem-like properties will form spherical colonies - termed mammospheres - when cultured in serum-free media on low-attachment substrates. Currently this assay is performed in commercially available 96-well trays with low-attachment surfaces. Here we report a novel microsystem that features on-chip mammosphere culture on low attachment surfaces. We have cultured mammospheres in this microsystem from well-studied human breast cancer cell lines. To enable the long-term culture of these unattached cells, we have integrated diffusion-based delivery columns that provide zero-convection delivery of reagents, such as fresh media, staining agents, or drugs. The multi-layer system consists of parallel cell-culture chambers on top of a low-attachment surface, connected vertically with a microfluidic reagent delivery layer. This design incorporates a reagent reservoir, which is necessary to reduce evaporation from the cell culture micro-chambers. The development of this microsystem will lead to the integration of mammosphere culture with other microfluidic functions, including circulating tumor cell recovery and high throughput drug screening. This will enable the cancer research community to achieve a much greater understanding of these tumor initiating cancer stem cells.

As a bio/chemical sensing technique, surface enhanced Raman spectroscopy (SERS) offers sensitivity comparable to that of fluorescence detection while providing highly specific information about the analyte. Although single molecule identification with SERS was demonstrated over a decade ago, today a need exists to develop practical solutions for point-of-sample and point-of-care SERS systems. In recent years, optofluidic SERS has emerged, in which microfluidic functions are integrated to improve the performance of SERS. Advancements in optofluidic SERS are leading towards portable analytical systems, but the devices are currently too expensive and too cumbersome for limited resource settings. Recently, we demonstrated the fabrication of SERS substrates by inkjet printing silver nanostructures onto paper. Using a low-cost commercial inkjet printer, we chemically patterned cellulose paper to form hydrophobic regions, which can control the aqueous sample on the paper microsystem. Additionally, we inkjet-printed silver nanoparticles with micro-scale precision to form SERS-active biosensors. Using these devices, we have been able to achieve detection limits comparable to conventional nanofabricated substrates. Furthermore, we leverage the fluidic properties to enhance the performance of the SERS devices while also enabling unprecedented ease of use. Paper dipsticks concentrate a relatively large sample volume into a small SERS-active detection region at the tip. Likewise, paper swabs collect samples from a large surface area and concentrate the collected molecules into a SERS sensor on the paper. We will summarize the progress in the fabrication and use of these paper-based optofluidic devices, and will describe their use in practical applications for point-of-sample detection.

We demonstrate a low-cost analytical technique using a dipstick with lateral-flow concentration and surface-enhanced Raman spectroscopy on an inkjet-printed plasmonic substrate. Lateral-flow concentration improves the detection limit by over 100X.

We report a surface-enhanced Raman spectroscopy device with on-chip mixing, integrated fiber optics, and nanofluidic concentration of Ag nanoclusters and adsorbed analytes. The device's detection limit is 100X better than open-channel microfluidics.

We demonstrate detection of 250 attomoles of Rhodamine 6G using an inkjet-fabricated SERS-active paper-based surface swab-dipstick. The fabrication simplicity and ease of use of this device is unprecedented for SERS-based analytics.

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages a nanoporous microfluidic matrix to improve the SERS detection performance by more than two orders of magnitude as compared to a typical open microfluidic channel. Although it is a growing trend to integrate optical biosensors into microfluidic channels, this basic combination has been detrimental to the sensing performance when applied to SERS. Recently, however, synergistic combinations between microfluidic functions and photonics (i.e., optofluidics) have been implemented that improve the detection performance of SERS. Conceptually, the simplest optofluidic SERS techniques reported to date utilize a single nanofluidic channel to trap nanoparticle-analyte conjugates as a method of preconcentration before detection. In this work, we leverage this paradigm while improving upon the simplicity by forming a 3D nanofluidic network with packed nanoporous silica microspheres in a microfluidic channel; this creates a concentration matrix that traps silver nanoclusters and adsorbed analytes into the SERS detection volume. With this approach, we are able to achieve a detection limit of 400 attomoles of Rhodamine 6G after only 2 min of sample loading with high chip-to-chip repeatability. Due to the high number of fluidic paths in the nanoporous channel, this approach is less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. In addition, fabrication of this microsystem is quite simple, as nanoscale fabrication is not necessary. Finally, integrated multimode fiber optic cables eliminate the need for optical alignment, and thus the device is relevant for portable and automated applications in the field, including point-of-sample and point-of-care detection. To illustrate a relevant field-based application, we demonstrate the detection of 12 ppb of the organophosphate malathion in water using the nanofluidic SERS microsystem.

A versatile waveguide-coupled optofluidic device using the liquid core optical ring resonator (LCORR) that can be operated with liquid of any refractive index (RI) is theoretically analyzed and experimentally demonstrated. The results confirm the confinement of resonant modes for all sample RIs, and reveal that confined modes in a high-RI core are excited by an external waveguide by resonant tunneling through the LCORR wall. It is further found that a thin wall must be used for effective interaction between the core mode and the waveguide. The results have important applications in optofluidic devices, including sensors, microfluidic lasers, and nonlinear optics.

We report the development of a novel, low-cost surface enhanced Raman spectroscopy (SERS) substrate that is fabricated by ink-jet-printing silver nanostructures into cellulose paper. Analysis of a liquid sample is performed by spotting a 1 microliter droplet onto the printed SERS substrate. The droplet is contained within a small area of the SERS substrate by ink-jet-printing hydrophobic barriers to define microfluidic boundaries. Using Rhodamine 6G as the analyte, we are able to measure a strong SERS fingerprint signal when only 10 femtomoles of analyte are applied to the paper.

We present ink-jet fabrication of optofluidic surface enhanced Raman spectroscopy (SERS) devices with no micro/nano-fabrication required. This novel, ultra-low-cost technique enables on-demand fabrication of SERS devices for lab and field use.

Optofluidic lasers are of particular interest for lab-on-a-chip-type devices, with broad spectral tunability, convenient microfluidic integration, and a small footprint. Optofluidic ring resonator (OFRR) lasers are advantageous in terms of size but typically generate nondirectional emission that is of minimal practical use. We introduce two unique geometries for soft-lithography-based OFRR lasers--side-coupled rings and spiral rings--both of which can be produced in polydimethyl siloxane substrates with contact molding. These rings utilize evanescent and direct butt-coupling, respectively, to effectively couple the OFRR laser emission into microfluidic channels. A laser threshold of a few to tens of microJ/mm(2) is achieved.

Despite decades of gains in biotechnology, currently much remains unknown about the molecular mechanisms of a number of deadly diseases. New tools are needed to speed discovery at the molecular level and to mitigate the complexity of studying the systems biology of disease. In the last decade, a number of advancements have been made in photonic biosensing aimed at developing a new generation of technologies for the study and early detection of disease. Label-free photonic biosensing techniques such as surface plasmon resonance imaging (SPRI), nanoporous silicon waveguides, interferometry, and optical resonators have achieved detection limits on par with conventional techniques while decreasing the size of the biosensing element and reducing the number of steps involved in the biosensing procedure. In addition, surface enhanced Raman spectroscopy (SERS) has been shown to detect molecules in extremely low numbers, even single molecules. In parallel to the development of a new generation of photonic biosensing techniques, advanced microfluidic technologies have emerged. The last decade has seen the development of automated micro-valves and micro-pumps, particle sorting, concentration gradient generation, and sample mixing. Also, photonic components, including waveguides and lenses, have been fabricated from soft-lithography materials. Synthesizing advanced microfluidic techniques with the new generation of photonic biosensing is likely to be the key to increasing throughput by increasing automation, decreasing equipment costs, and enabling multiplexing. This report will review examples of recent work in optofluidic biosensors and will discuss the opportunities for advancing research related to disease at the molecular level utilizing optofluidic biosensing technologies.

Microfluidic lasers, which utilize liquid as a gain medium, are of great interest for lab-on-a-chip devices due to their small size, tunability, and cost-effectiveness. We demonstrate a soft-lithography-based opto-fluidic ring resonator (OFRR) laser which can be produced in arrays of identical rings in polydimethyl siloxane (PDMS). The PDMS structures are produced from a silicon mold fabricated using reactive ion etching (RIE) and are both robust and reusable. Using rhodamine 6G in a tetraethylene glycol (TEG) dye solvent provides enough refractive index contrast with PDMS to generate a multimode lasing signal from rings 200 to 400 microns in diameter and lasing thresholds of 2.7 μJ/mm 2 centered around 580 nm. These rings are coupled to liquid waveguides which conveniently direct the lasing emission to other on-chip devices. Since the rings and waveguides are not in fluidic contact, many rings may potentially be coupled into a single waveguide for multi-color emission. Separating the ring and waveguide fluidics also prevents unwanted absorption of the lasing signal by extra dye molecules.

Nucleic acids (such as DNA and RNA) play a central role in biological systems, carrying instructions that govern the function of all living organisms. Many of the most promising applications of lab-on-a-chip technology, such as infectious disease detection, gene expression profiling and DNA sequencing and amplification involve processing these carriers of genetic information. While decades of work has been dedicated to moving processes like these on-chip, the important precursor steps of isolating nucleic acids from samples have been largely overlooked. Fully integrating the sample preparation on-chip will increase processing speed while reducing operator error and sample contamination as well as labor and reagent related costs. To date the typical approach for implementing nucleic acid purification in a microchannel format is to translate the commonly used solid phase extraction method which relies on reagent changes to achieve immobilization and release of DNA. By using electrostatic control of DNA we are able to avoid the reagents (including PCR inhibitors) and complicated valving that this approach requires. We microfabricate chips using photolithography to create gold microelectrodes on glass substrates, upon which we bond a PDMS microchannel which is fabricated using soft lithography. Through COMSOL simulations we optimize our electrode geometry to maximize DNA capture efficiency. Designs are qualitatively evaluated using fluorescence microscopy. For quantification, we measure both the DNA removed from a sample (capture efficiency) and the DNA that can be released back into solution (elution efficiency) using UV spectrophotometry. By dramatically improving the ease and flexibility of nucleic acid purification, this enhanced microfluidic module can be integrated into existing microfluidic designs, improving how researchers approach on-chip nucleic acid processing and diagnostics.

In this work, we integrate a capillary ring resonator (CRR) biosensor with microfluidics based on polydimethylsiloxane (PDMS) with soft-lithography. The CRR performs refractive index detection for chemical or biomolecule detection, and has previously been used for protein and DNA sequence detection. In initial implementations of the CRR there are practical difficulties in fabrication and integration with other microfluidic functions. Microfluidic systems have been demonstrated for many on-chip capabilities such as sample/ reagent mixing, concentration gradient preparation, cell culture and lysis, and pre-concentration of samples. Combining the CRR biosensor with microfluidic systems will allow for creating a single chip microfluidic system, which will eliminate complex steps and complicated techniques.

To improve the sensitivity of microfluidic SERS, we constructed an in-situ photopolymerized monolith in a microchannel with embedded silver nanoclusters. Rhodamine-6G was detected easily after passing only 400 fmoles of analyte through the microchannel.

We develop a versatile integrated opto-fluidic ring resonator (OFRR) dye laser that can be operated regardless of the refractive index (RI) of the liquid. The OFRR is a micro-sized glass capillary with a wall thickness of a few micrometers. When the liquid in the core has an RI lower than that of the capillary wall (n=1.45), the capillary circular cross-section forms the ring resonator and supports the whispering gallery modes (WGMs) that interact evanescently with the gain medium in the core. When the core RI is higher than that of the wall, the WGMs exist at the core/wall interface. In both cases, the WGMs can have extremely high Q-factor (109), providing excellent optical feedback for low-threshold lasing. In this paper, we analyze the OFRR laser for various core RI's and then we demonstrate the R6G laser when the dye is in ethanol (n=1.36), chloroform (n=1.445), and quinoline (n=1.626). The lasing threshold of 25 nJ/mm(2) is achieved, two to three orders of magnitude lower than the previous work in microfluidic lasers. We further show that the laser emission can be efficiently out-coupled via an optical waveguide in touch with the OFRR for both high and low RI liquid core, allowing for easy guiding and delivery of the laser light.

The development of surface enhanced Raman scattering (SERS) detection has made Raman spectroscopy relevant for highly sensitive labon- a-chip bio/chemical sensors. Despite the tremendous benefit in specificity that a Raman-based sensor can deliver, development of a lab-on-a- chip SERS tool has been limited thus far. In this work, we utilize an optofluidic ring resonator (OFRR) platform to develop a SERS-based detection tool with integrated microfluidics. The liquid core optical ring resonator (LCORR) serves both as the microfluidic sample delivery mechanism and as a ring resonator, exciting the metal nanoclusters and target analytes as they pass through the channel. Using this OFRR approach and R6G as the analyte, we have achieved a measured detection limit of 400 pM. The measured Raman signal in this case is likely generated by only a few hundred R6G molecules, which foreshadows the development of a SERS-based lab-on-a-chip bio/chemical sensor capable of detecting a low number of target analyte molecules.

We theoretically and experimentally analyze the biomolecule detection capability of the liquid core optical ring resonator (LCORR) as a label-free bio/chemical sensor. We first establish a simple and general linear relationship between the LCORR's bulk refractive index sensitivity (BRIS) and its response to molecule deposition onto the surface, which enables us to easily characterize the LCORR sensing performance. Then, biosensing experiments are performed with bovine serum albumin (BSA) and LCORRs of various BRISs. The experimental results are in good agreement with the theoretical prediction. Further analysis shows that the LCORR is capable of detecting BSA below 10 pM with sub-picogram/mm2 mass detection limit.

We demonstrate a new method to tune the resonance of whispering gallery modes in a fused silica optical microsphere resonator by removing atomic layers from the sphere surface with low concentrations of hydrofluoric acid. Our results show that the WGMs can be tuned over 660 pm (430 GHz), more than one free spectral range of the microsphere resonator, with a tuning precision better than 0.2 pm (130 MHz). Both atomic force microscope images and a Q-factor measurement performed in air suggest that no additional degradation in Q-factor due to surface roughness is introduced during this etching process.

Optical label-free detection prevents the cost and complexity of fluorescence and radio labeling while providing accurate quantitative and kinetic results. We have developed a new optical label-free sensor called the liquid core optical ring resonator (LCORR). The LCORR integrates optical ring resonator sensors into the microfluidic delivery system by using glass capillaries with a thin wall. The LCORR is capable of performing refractive index detection on liquid samples, as well as bio/chemical analyte detection down to detection limits on the scale of pg/mm2 on a sensing surface.

The optofluidic ring resonator (OFRR) is a label-free refractive index biosensing platform that uses a glass capillary as a microfluidic sample delivery mechanism with an inherently integrated optical ring resonator. The device delivers molecular detection limits on par with the top-performing commercial label-free sensing technologies while reducing cost and complexity compared with fluorescence-based biosensing techniques as well as other label-free devices. The OFRR has been shown to quantify target molecules in a sample through specific biomolecular recognition events at the surface of the capillary. Examples presented here include protein detection, DNA sequence detection, and pathogen detection. The design of the OFRR makes it well-suited for integration into a lab-on-a-chip platform that will have significant implications for the diagnosis and understanding of disease.

The optofluidic ring resonator (OFRR) is a novel gas sensing technology platform. In an OFRR gas sensor, the OFRR interior surface is coated with a layer of vapor-sensitive polymer. The interaction between the polymer and the gas molecules flowing through the OFRR results in a change in polymer refractive index and thickness, which can be detected by the circulating waveguide modes supported by the circular cross section of the OFRR. Due to the excellent fluidics of a capillary, the OFRR is capable of detecting chemical vapors rapidly with very low sample volume. In addition, the OFRR is highly compatible with gas chromatography (GC) and is a promising platform for development of micro-GC (μGC) with unique multipoint, on-column detection capability. In this chapter, we will discuss the fundamental operational principles of the OFRR gas sensor, followed by examples of rapid detection of several representative vapor analytes. The development of an OFRR-based μGC system and its applications in explosive separation and detection will also be presented.

We discuss versatile, miniaturized optofluidic ring resonator (OFRR) dye lasers that can be operated regardless of the refractive index of the liquid. The OFRR is a piece of a thin-walled fused silica capillary that integrates the photonic ring resonator with microfluidics. In an OFRR dye laser, the active lasing materials (such as dyes) are passed through the capillary whereas the circular cross section forms a ring resonator and supports whispering gallery modes that provide optical feedback for lasing. Because of the high Q-factors extremely low lasing threshold is achieved (25 nJ/mm2). The operation wavelength can conveniently be changed by using different dyes and fine-tuned with solvent. The OFRR laser is excited through direct excitation or through efficient energy transfer. The laser can be efficiently out-coupled through a fiber taper in contact with the capillary, thus providing easy guiding for the laser emission. Theoretical analysis and experimental results for OFRR lasers are presented.

Optical ring resonators have been investigated for a number of interesting devices, including dye lasers and sensors. However, in general, these devices can only operate on liquid samples with a low refractive index (RI) because the whispering gallery modes (WGMs) are bound in the resonator through total internal reflection at the resonator/sample boundary. We recently introduced a new opto-fluidic ring resonator (OFRR) that uses a thin-walled capillary to deliver the sample through an array of ring resonators contained within the circular cross-section of the capillary. Thus, in the OFRR, the WGM is bound at the outer surface while the evanescent field interacts with the sample at the inner surface. Therefore, the OFRR can operate on samples of lower and higher RI than the capillary material. This unique feature, in combination with the OFRR's practical fluidic delivery design and its simplicity make it an attractive opto-fluidic device for sensors, lasers, and other applications. We analyze the OFRR's capability to support WGMs that are excited externally through fiber tapers and that interact with the sample inside. Using a quantum mechanical analogy, we show that for liquid cores with a higher RI than the capillary material, two coupled propagating waves exist that enable WGMs inside the liquid core to be excited by a fiber taper outside the OFRR, across a few microns. We experimentally verify our analysis by demonstrating refractometric sensors and dye lasers with core RIs lower and higher than the capillary.

We develop rapid chemical vapor sensors and micro gas chromatography (μGC) analyzers based on the optofluidic ring resonator (OFRR). An OFRR is a micro-sized thin-walled glass capillary; the circular cross-section of the capillary acts as an optical ring resonator while the whispering gallery modes or circulating waveguide modes (WGMs) supported by the ring resonator interact with the vapor samples passing through the capillary. The OFRR interior surface is coated with a vapor-sensitive polymer. The analyte and polymer interaction causes the polymer refractive index (RI) and the thickness to change, which is detected as a WGM spectral shift. Owing to the excellent fluidics, the OFRR vapor sensor exhibits sub-second detection and recovery time with a flow rate of 1 mL/min. On-column separation and detection in the OFRR based μGC system is also demonstrated, showing efficient separation of vapor mixtures and presenting highly reproducible retention time for the individual analyte. Compared to the conventional GC system, the OFRR μGC has the advantage of small size, rapid response, and high selectivity over a short length of column.

Opto-fluidic ring resonators (OFRR) integrate microfluidics with state-of-the-art photonics sensing technology. Here we developed a label-free OFRR biosensor with bacteriophage as a biorecognition probe for streptavidin detection. A detection limit of 0.17 nM was achieved.