Compact characterization of liquid absorption and emission spectra using linear variable filters integrated with a CMOS imaging camera

Compact Fluorescence Spectrometer with Built-In In-Line Calibration: Application to Detect Dissolved Organic Matter in Water

Fluorescence spectrometer (FS) is widely used for component analysis because each fluorescing material has its own characteristic spectrum. However, the spectral calibration is complicated and bulky. Herein, an in-line spectral calibration sheet (ISCS) was proposed in which a narrow band-pass filter and a linear variable filter (LVF) were integrated on a metal plate. By moving the ISCS, the transmitted excitation light power (TEP) as well as fluorescence spectrum can be seamlessly scanned, and the TEP can be used for in-line spectral calibration. A compact FS apparatus based on UV-LED excitation, metal capillary (MC) and ISCS was fabricated (i.e., ISCS-FS), and the ISCS-FS apparatus was applied to detect sodium humate in water. By employing TEP calibration, both the primary inner filter effect (PIFE) and the drift in the optical power of UV-LED can be simultaneously compensated. The linear correlation coefficient of signal concentration was improved from 0.89 to 0.998, and the relative standard deviation (RSD) of replicated detection was improved from 3 to 0.7%. A detection limit of concentration (DLC) of 1.3 μg/L was realized, which is 15-fold lower than that of a commercial FS apparatus (20 μg/L). The DLC is even comparable with that (0.5-4 μg/L) of commercial total organic carbon (TOC) analyzers, which are bulky and expensive. The linear correlation between the measurement results of ISCS-FS and commercial TOC analyzers can reach a good value of 0.94.

Folded beam path architecture for highly efficient filter-based spectral sensors

This paper demonstrates a method to significantly enhance the detection efficiency of filter-based spectral sensors without the use of additional dichroic optics for spectral preselection. The fundamental principle is that light reflected from one interference filter or filter segment can be used consecutively, reducing the overall system losses. The proof-of-concept is presented using two compact optical modules. The first module uses 10 individual filters between 520 and 800 nm, and the second is capable of continuous spectrum acquisition between 450 and 825 nm using a linear variable filter (LVF) as a key element. An efficiency increase factor of up to approximately 100 compared to a common system, where the entire LVF is directly illuminated, was demonstrated.

A Compact Detection Platform Based on Gradient Guided-Mode Resonance for Colorimetric and Fluorescence Liquid Assay Detection

This paper presents a compact spectral detection system for common fluorescent and colorimetric assays. This system includes a gradient grating period guided-mode resonance (GGP-GMR) filter and charge-coupled device. In its current form, the GGP-GMR filter, which has a size of less than 2.5 mm, can achieve a spectral detection range of 500-700 nm. Through the direct measurement of the fluorescence emission, the proposed system was demonstrated to detect both the peak wavelength and its corresponding intensity. One fluorescent assay (albumin) and two colorimetric assays (albumin and creatinine) were performed to demonstrate the practical application of the proposed system for quantifying common liquid assays. The results of our system exhibited suitable agreement with those of a commercial spectrometer in terms of the assay sensitivity and limit of detection (LOD). With the proposed system, the fluorescent albumin, colorimetric albumin, and colorimetric creatinine assays achieved LODs of 40.99 and 398 and 25.49 mg/L, respectively. For a wide selection of biomolecules in point-of-care applications, the spectral detection range achieved by the GGP-GMR filter can be further extended and the simple and compact optical path configuration can be integrated with a lab-on-a-chip system.

A coma-free super-high resolution optical spectrometer using 44 high dispersion sub-gratings

Unlike the single grating Czerny-Turner configuration spectrometers, a super-high spectral resolution optical spectrometer with zero coma aberration is first experimentally demonstrated by using a compound integrated diffraction grating module consisting of 44 high dispersion sub-gratings and a two-dimensional backside-illuminated charge-coupled device array photodetector. The demonstrated super-high resolution spectrometer gives 0.005 nm (5 pm) spectral resolution in ultra-violet range and 0.01 nm spectral resolution in the visible range, as well as a uniform efficiency of diffraction in a broad 200 nm to 1000 nm wavelength region. Our new zero-off-axis spectrometer configuration has the unique merit that enables it to be used for a wide range of spectral sensing and measurement applications.

Mixed-gas CH4/CO2/CO detection based on linear variable optical filter and thermopile detector array

This paper presents the design, fabrication, and characterization of a middle-infrared (MIR) linear variable optical filter (LVOF) and thermopile detectors that will be used in a miniaturized mixed gas detector for CH₄/CO₂/CO measurement. The LVOF was designed as a tapered-cavity Fabry-Pérot optical filter, which can transform the MIR continuous spectrum into multiple narrow band-pass spectra with peak wavelength in linear variation. Multi-layer dielectric structures were used to fabricate the Bragg reflectors on the both sides of tapered cavity as well as the antireflective film combined with the function of out-of-band rejection. The uncooled thermopile detectors were designed and fabricated as a multiple-thermocouple suspension structure using micro-electro-mechanical system technology. Experimentally, the LVOF exhibits a mean full-width-at-half-maximum of 400 nm and mean peak transmittance of 70% at the wavelength range of 2.3~5 μm. The thermopile detectors exhibit a responsivity of 146 μV/°C at the condition of room temperature. It is demonstrated that the detectors can achieve the quantification and identification of CH₄/CO₂/CO mixed gas.

Ultrahigh-resolution spectrometer based on 19 integrated gratings

Optical spectrometers play a key role in acquiring rich photonic information in both scientific research and a wide variety of applications. In this work, we present a new spectrometer with an ultrahigh resolution of better than 0.012 nm/pixel in the 170–600 nm spectral region using a grating-integrated module that consists of 19 subgratings without any moving parts. By using two-dimensional (2D) backsideilluminated complementary metal-oxide-semiconductor (BSI-CMOS) array detector technology with 2048 × 2048 pixels, a high data acquisition speed of approximately 25 spectra per second is achieved. The physical photon-sensing size of the detector along the one-dimensional wavelength direction is enhanced by a factor of 19 to approximately 428 mm, or 38912 pixels, to satisfy the requirement of seamless connection between two neighboring subspectral regions without any missing wavelengths throughout the entire spectral region. As tested with a mercury lamp, the system has advanced performance capabilities characterized by the highest k parameter reported to date, being approximately 3.58 × 10⁴, where k = (working wavelength region)/(pixel resolution). Data calibration and analysis as well as a method of reducing background noise more efficiently are also discussed. The results presented in this work will stimulate further research on precision spectrometers based on advanced BSI-CMOS array detectors in the future.

Portable Spectroscopy

Until very recently, handheld spectrometers were the domain of major analytical and security instrument companies, with turnkey analyzers using spectroscopic techniques from X-ray fluorescence (XRF) for elemental analysis (metals), to Raman, mid-infrared, and near-infrared (NIR) for molecular analysis (mostly organics). However, the past few years have seen rapid changes in this landscape with the introduction of handheld laser-induced breakdown spectroscopy (LIBS), smartphone spectroscopy focusing on medical diagnostics for low-resource areas, commercial engines that a variety of companies can build up into products, hyphenated or dual technology instruments, low-cost visible-shortwave NIR instruments selling directly to the public, and, most recently, portable hyperspectral imaging instruments. Successful handheld instruments are designed to give answers to non-scientist operators; therefore, their developers have put extensive resources into reliable identification algorithms, spectroscopic libraries or databases, and qualitative and quantitative calibrations. As spectroscopic instruments become smaller and lower cost, "engines" have emerged, leading to the possibility of being incorporated in consumer devices and smart appliances, part of the Internet of Things (IOT). This review outlines the technologies used in portable spectroscopy, discusses their applications, both qualitative and quantitative, and how instrument developers and vendors have approached giving actionable answers to non-scientists. It outlines concerns on crowdsourced data, especially for heterogeneous samples, and finally looks towards the future in areas like IOT, emerging technologies for instruments, and portable hyphenated and hyperspectral instruments.

A High-Performance Spectrometer with Two Spectral Channels Sharing the Same BSI-CMOS Detector

Optical spectrometers play an important role in modern scientific research. In this work, we present a two-channel spectrometer with a pixel resolution of better than 0.1 nm/pixel in the wavelength range of 200 to 950 nm and an acquisition speed of approximately 25 spectra per second. The spectrometer reaches a high k factor which characterizes the spectral performance of the spectrometer as k = (working wavelength region)/(pixel resolution) = 7500. Instead of using mechanical moving parts in traditional designs, the spectrometer consists of 8 integrated sub-gratings for diffracting and imaging two sets of 4-folded spectra on the upper and lower parts, respectively, of the focal plane of a two-dimensional backside-illuminated complementary metal-oxide-semiconductor (BSI-CMOS) array detector, which shows a high peak quantum efficiency of approximately 90% at 400 nm. In addition to the advantage of being cost-effective, the compact design of the spectrometer makes it advantageous for applications in which it is desirable to use the same two-dimensional array detector to simultaneously measure multiple spectra under precisely the same working conditions to reduce environmental effects. The performance of the finished spectrometer is tested and confirmed with an Hg-Ar lamp.

Point-of-use detection of ascorbic acid using a spectrometric smartphone-based system

A rapid and portable analytical methodology has been developed for ascorbic acid (Vitamin C) quantification from aqueous samples using a spectrometric smartphone-based system for the first time. The method employs point-of-use approaches both for sample preparation and sample measurement, demonstrating the capability for mobile quality control of pharmaceutical and food products. Our approach utilizes an oxidation-reduction reaction between ascorbic acid and methylene blue, followed by a dispersive liquid-liquid microextraction (DLLME) to extract the aqueous-phase methylene blue into organic media. Then, a back-extraction procedure is employed to transfer the methylene blue to aqueous media, followed by analysis of the sample's absorption spectrum using the spectrometric smartphone-based system. The DLLME and back-extraction procedures are optimized by use of a two-step multivariate optimization strategy. Finally, vitamin C supplements and orange juice are used as real-world samples to assess the applicability of the smartphone-based method, which is successfully compared with the standard laboratory-based approach.

Mobile Platform for Multiplexed Detection and Differentiation of Disease-Specific Nucleic Acid Sequences, Using Microfluidic Loop-Mediated Isothermal Amplification and Smartphone Detection

New tools are needed to enable rapid detection, identification, and reporting of infectious viral and microbial pathogens in a wide variety of point-of-care applications that impact human and animal health. We report the design, construction, and characterization of a platform for multiplexed analysis of disease-specific DNA sequences that utilizes a smartphone camera as the sensor in conjunction with a hand-held "cradle" that interfaces the phone with a silicon-based microfluidic chip embedded within a credit-card-sized cartridge. Utilizing specific nucleic acid sequences for four equine respiratory pathogens as representative examples, we demonstrated the ability of the system to utilize a single 15 μL droplet of test sample to perform selective positive/negative determination of target sequences, including integrated experimental controls, in approximately 30 min. Our approach utilizes loop-mediated isothermal amplification (LAMP) reagents predeposited into distinct lanes of the microfluidic chip, which when exposed to target nucleic acid sequences from the test sample, generates fluorescent products that when excited by appropriately selected light emitting diodes (LEDs), are visualized and automatically analyzed by a software application running on the smartphone microprocessor. The system achieves detection limits comparable to those obtained by laboratory-based methods and instruments. Assay information is combined with the information from the cartridge and the patient to populate a cloud-based database for epidemiological reporting of test results.