Performance of a simple UV LED light source in the capillary electrophoresis of inorganic anions with indirect detection using a chromate background electrolyte

Light emitting diodes (LEDs) are known to be excellent light sources for detectors in liquid chromatography and capillary electromigration separation techniques, but to date only LEDs emitting in the visible range have been used. In this work, a UV LED was investigated as a simple alternative light source to standard mercury or deuterium lamps for use in indirect photometric detection of inorganic anions using capillary electrophoresis with a chromate background electrolyte (BGE). The UV LED used had an emission maximum at 379.5 nm, a wavelength at which chromate absorbs strongly and exhibits a 47% higher molar absorptivity than at 254 nm when using a standard mercury light source. The noise, sensitivity and linearity of the LED detector were evaluated and all exhibited superior performance to the mercury light source (up to 70% decrease in noise, up to 26.2% increase in sensitivity, and over 100% increase in linear range). Using the LED detector with a simple chromate-diethanolamine background electrolyte, limits of detection for the common inorganic anions, Cl-, NO3-, SO4(2-), F- and PO4(3-) ranged from 3 to 14 microg L(-1), using electrostatic injection at -5 kV for 5 s.     

Low-picomolar limits of detection using high-power light-emitting diodes for fluorescence

Fluorescence detectors are ever more frequently being used with light-emitting diodes (LEDs) as the light source. Technological advances in the solid-state lighting industry have produced LEDs which are also suitable tools in analytical measurements. LEDs are now available which deliver 700 mW of radiometric power. While this greater light power can increase the fluorescence signal, it is not trivial to make proper use of this light. This new generation of LEDs has a large emitting area and a highly divergent beam. This presents a classic problem in optics where one must choose between either a small focused light spot, or high light collection efficiency. We have selected for light collection efficiency, which yields a light spot somewhat larger than the emitting area of the LED. This light is focused onto a flow cell. Increasing the detector cell internal diameter (i.d.) produces gains in (sensitivity)3. However, since the detector cell i.d. is smaller than the LED spot size, scattering of excitation light towards the detector remains a significant source of background signal. This can be minimized through the use of spectral filters and spatial filters in the form of pinholes. The detector produced a limit of detection (LOD) of 3 pM, which is roughly three orders of magnitude lower than other reports of LED-based fluorescence detectors. Furthermore, this LOD comes within a factor of six of much more expensive laser-based fluorescence systems. This detector has been used to monitor a separation from a gel filtration column of fluorescently labeled BSA from residual labeling reagent. The LOD of fluorescently labeled BSA is 25 pM.     

A low-cost light-emitting diode induced fluorescence detector for capillary electrophoresis based on an orthogonal optical arrangement

In this work, a simple and low-cost miniaturized light-emitting diode induced fluorescence (LED-IF) detector based on an orthogonal optical arrangement for capillary electrophoresis (CE) was developed, using a blue concave light-emitting diode (LED) as excitation source and a photodiode as photodetector. A lens obtained from a waste DVD-ROM was used to focus the LED light beam into an ∼80 μm spot. Fluorescence was collected with an ocular obtained from a pen microscope at 45° angle, and passed through a band-pass filter to a photodiode detector. The performance of the LED-IF detector was demonstrated in CE separations using sodium fluorescein and fluorescein isothiocyanate (FITC)-labeled amino acids as model samples. The limit of detection for sodium fluorescein was 0.92 μM with a signal-to-noise ratio (S/N) of 3. The total cost of the LED-IF detector was less than $ 50.     

New Phosphors for White LEDs

White light-emitting diodes (WLEDs) have matched the emission efficiency of florescent lights and will rapidly spread as light source for homes and offices in the next 5 to 10 years. WLEDs provide a light element having a semiconductor light emitting layer (blue or UV LEDs) and photoluminescence phosphors. GaN-based highly efficient blue InGaN LEDs combined with phosphors can produce white light. These solid-state LED lamps have a number of advantages over conventional incandescent bulbs and halogen lamps, such as high efficiency to convert electrical energy into light, reliability, and long operating lifetime (about 100,000 hours). For the purpose of development of high energy-efficient white light sources, we need to produce highly efficient new phosphors, which can absorb excitation energy from blue or UV LEDs and generate emissions.In this paper, we investigate the development of blue or UV LEDs by the appropriate combination of new phosphors which can lead us to obtain high brightness white light. The criteria of choosing the best phosphors, for blue (380-450 nm) and UV (360-400 nm) LEDs, strongly depends on the absorption and emission of the phosphors. Moreover, the balance light between the light emission from blue LEDs and the yellow YAG:Ce,Gd phosphor is important to obtain white light with high color temperature. The phosphors with high efficiency which can be excited by UV LEDs are important to obtain the white light with high color rendering index.     

Construction of a light-emitting diode fluorescence detector for high-performance liquid chromatography and its application to fluorometric determination of L-3-hydroxybutyrate

This study employed a new light source, a light-emitting diode (LED), for fluorescence detection of high-performance liquid chromatography to measure the concentration of trace constituents in biological fluids. Using l-3-hydroxybutyrate ( l-3HB) as a tested trace compound, the function of the new system was compared with that of the current commercially available model. A detailed schematic diagram of the path of the detection rays in the LED detector is given. A voltage-stabilizer for the drive circuit was designed with an input of 10 V and an output of 8 V, and another voltage regulator was used to maintain a constant 8 V. Then the regulator was used to set the output voltage for the LED at 2.8 V by two external resistors. Replacing the xenon lamp with LED, this system provided higher photon density and a narrow spectrum at a wavelength of 491 nm. At room temperature (22.1°C), the average temperature of six places in the chamber of LED detector was 22.1°C compared with 51.1°C in the xenon detector. The spectra of the excitation light sources were measured. Compared with the xenon lamp, approximately 1.32 times higher excitation intensity was obtained by the LED source. The accuracy of detection of l-3HB in 50 μL of rat serum was 99.85-100.85%, and the intra-day and inter-day precision values were within 8.99 and 13.90%, respectively. The limit of detection of l-3HB was approximately 0.73 μM (signal-to-noise ratio 3). The sensitivity of the proposed LED detector was comparable to that of traditional fluorescence detectors using xenon arc lamps; however, the cost and operating temperature of LED lamps were far lower. This assay system could be further used to detect trace constituents in various samples.     

High-power blue/UV light-emitting diodes as excitation sources for sensitive detection

With advances in III-V nitride manufacturing processes, high-power light-emitting diode (LED) chips in the blue and UV wavelengths are now commercially available at reasonable cost and can be used as excitation sources in optical sensing. We describe the use of these high-power blue and UV LEDs for sensitive fluorescence detection, including chip-based flow cytometry, capillary electrophoresis (CE), and single-molecule imaging. By using a blue LED with a focusable power of approximately 40 mW as the excitation source for fluorescent beads, we demonstrate a simple chip-based bead sorter capable of enriching the concentration of green fluorescent beads from 63% to 95%. In CE experiments, we show that a mixture of analyte solution containing 30 nM 6-carboxyrhodamine 6G and 10 nM fluorescein can be separated and detected with excellent signal-to-noise ratio (approximately 17 for 10 nM fluorescein) using the collimated emission from a blue LED; the estimated mass detection limit was approximately 200 zmol for fluorescein. We also demonstrated ultrasensitive fluorescence imaging of single rhodamine 123 molecules and individual lambda-DNA molecules. At a small fraction of the cost of an Ar+ laser, high-power blue and UV LEDs are effective alternatives for lasers and arc lamps in fluorescence applications that demand portability, low cost, and convenience.     

LED–LED portable oxygen gas sensor

A portable instrument for oxygen determination, based on the quenching of phosphorescent octaethylporphyrin by gaseous O₂, has been developed using the fluorimetric paired emitter–detector diode technique (FPEDD). The instrument configuration consists of two light-emitting diodes (LEDs) facing each other, with an interchangeable support containing a phosphorescent membrane in between, in which one of the LEDs is used as the light source (emitter LED) and the other, working in reverse bias mode, as the light detector. The feasibility of using a LED as a luminescence detector is studied. Its small size enables integration of the instrument into a portable measurement system. A systematic study of the system capabilities as a portable instrument was performed to optimize range, sensitivity, short term and long term stability, dynamic behaviour, effect of temperature and humidity, and temporal drift.     

Light emitting diode-based detectors Absorbance, fluorescence and spectroelectrochemical measurements in a planar flow-through cell

Light emitting diodes (LEDs) were first used for chemical analysis three decades ago. They are finally making their appearance in commercial analytical systems and dedicated detectors. LEDs are the most energy-efficient means of producing monochromatic light, and provide a concentrated small cool emitter ideal for miniature analytical devices. Although they rank behind fluorescent and halogen discharge lamps in total conversion efficiency (lm/W), new efficiency records are being set every year such that by next decade broadband (white) LED sources are not only likely in analytical instrumentation, but for general illumination. This paper begins with a review of analytical use of LEDs that has been advanced in the last decade. LED-based absorbance measurement and its use in pedagogy, titrations, in providing immunity to refractive index and turbidity effects, in field and process analysis, in capillary electrophoresis (CE), in liquid–liquid extraction systems, in film and drop-based analytical systems and with liquid core waveguides (LCWs) are discussed. LED-based fluorescence and spectroelectrochemical detection follows next. Multipurpose LED-based analytical instrumentation and special analytical applications and general applications are discussed. A listing of (mostly web-based) resources for fabricating LED-based detectors is then provided. Detector circuits and available components are considered and different modes of driving LEDs are compared. The temperature dependence of LED characteristics and strategies to ameliorate this problem are discussed.

The review and general resource material is followed with the construction details, operation and performance observed for a simple-to-fabricate multipurpose cell that allows simultaneous multiwavelength absorbance, fluorescence and spectroelectrochemical detection.     

Feasibility of light-emitting diode uses for annular reactor inner-coated with TiO2 or nitrogen-doped TiO2 for control of dimethyl sulfide

Limited environmental pollutants have only been investigated for the feasibility of light‐emitting diodes (LED) uses in photocatalytic decomposition (PD). The present study investigated the applicability of LEDs for annular photocatalytic reactors by comparing PD efficiencies of dimethyl sulfide (DMS), which has not been investigated with any LED‐PD system, between photocatalytic systems utilizing conventional and various LED lamps with different wavelengths. A conventional 8 W UV/TiO₂ system exhibited a higher DMS PD efficiency as compared with UV‐LED/TiO₂ system. Similarly, a conventional 8 W visible‐lamp/N‐enhanced TiO₂ (NET) system exhibited a higher PD efficiency as compared with six visible‐LED/NET systems. However, the ratios of PD efficiency to the electric power consumption were rather high for the photocatalytic systems using UV‐ or visible‐LED lamps, except for two LED lamps (yellow‐ and red‐LED lamps), compared to the photocatalytic systems using conventional lamps. For the photocatalytic systems using LEDs, lower flow rates and input concentrations and shorter hydraulic diameters exhibited higher DMS PD efficiencies. An Fourier‐transformation infrared analysis suggested no significant absorption of byproducts on the catalyst surface. Consequently, it was suggested that LEDs can still be energy‐efficiently utilized as alternative light sources for the PD of DMS, under the operational conditions used in this study.     

Performance of methylphenyl hydrogen-containing silicone oils for LED encapsulation

Three-dimensional light-emitting diodes (LEDs) are the preferred light source for bulb lamps due to their 360° illumination. Addition curing silicone resins have usually been used to encapsulate LEDs. LED-filament encapsulation needs silicone resins having high thermal performance and thixotropy. Herein, a low refractive index and highly anti-thermal aging silicone oil was synthesized by hydrolysis and co-condensation method, and used to prepare an encapsulation material for high-power LED-filament. The cured silicone materials were characterized by thermogravimetric analysis (TGA) and thermal aging test under 180°C and 250°C. The results demonstrated that the thermal stability of the cured silicone resins with short-chain phenyl silicone as a crosslinker was higher than that with long-chain methyl silicone oil crosslinker. Owing to the excellent thermal stability, appropriate hardness, high transparency and photoelectric efficiency, this LED encapsulation material is a promising candidate for high-power LED package.     

Optical fiber light-emitting diode-induced fluorescence detection for capillary electrophoresis

A highly sensitive optical fiber light-emitting diode (LED)-induced fluorescence detector for CE has been constructed and evaluated. In this detector, a violet or blue LED was used as the excitation source and an optical fiber with 40 microm OD was used to transmit the excitation light. The upper end of the fiber was inserted into the separation capillary and was situated right at the detection window. Fluorescence emission was collected by a 40 x microscope objective, focused on a spatial filter, and passed through a cutoff filter before reaching the photomultiplier tube. Output signals were recorded and processed with a computer using in-house written software. The present CE/fluorescence detector deploys a simple and inexpensive optical system that requires only an LED as the light source. Its utility was successfully demonstrated by the separation and determination of amino acids (AAs) labeled with naphthalene-2,3-dicarboxaldehyde (NDA) and FITC. Low detection limits were obtained ranging from 17 to 23 nM for NDA-tagged AAs and 8 to 12 nM for FITC-labeled AAs (S/N=3). By virtue of such valuable features as low cost, convenience, and miniaturization, the presented detection scheme was proven to be attractive for sensitive fluorescence detection in CE.     

Design and performance of a light-emitting diode detector compatible with a commercial capillary electrophoresis instrument

Indirect photometric detection in capillary electrophoresis (CE) has been predominantly performed in the UV region, in part due to a lack of suitable high-intensity and low-noise light sources in the visible spectral region. A new photometric detector based on light-emitting diodes (LEDs) as light sources and compatible with a commercially available CE instrument has been designed and constructed and its performance evaluated. The utility of this detector was successfully demonstrated by the indirect photometric detection of anions using a dye as probe and absorbance measured in the visible region. The detector exhibited very low baseline noise (around 0.03 mAU), stable output, and improved upper limit of detection linearity (502 mAU) compared with previously used LED detectors. The detector was tested for indirect detection of anions separated with an electrolyte containing 4 mM Orange G as the indirect detection probe, 10 mM histidine as an isoelectric buffer, and 0.05% hydroxypropylmethylcellulose to suppress the electroosmotic flow. Extremely low detection limits were obtained ranging from 0.16-0.36 microM (excluding chloride 0.56 microM), with separation efficiencies in the range of 154,000-274,000 theoretical plates.     

A lamp light-emitting diode-induced fluorescence detector for capillary electrophoresis

A light-emitting diode-induced fluorescence detector (LED-FD) for capillary electrophoresis was constructed and evaluated. A lamp LED with an enhanced emission spectrum and a band pass filter was used as the excitation light source. Refractive index matching fluid (RIMF) was used in the detection cell to reduce scattering light and the noise level. The limit of detection (LOD) for fluorescein was 1.5 nM (SNR=3). The system exhibited linear responses in the range of 1 x 10(-8) to 5 x 10(-6)M (R=0.999). Application of the lamp LED-FD for the analysis of FITC-labeled ephedra herb extract by capillary electrophoresis was demonstrated.     

Light‐emitting‐diode‐induced chemiluminescence detection for capillary electrophoresis

In this work, an LED‐induced‐chemiluminescence (LED‐CL) system was developed to extend the application of CL detection in CE. In the LED‐CL, the analyte photooxidizes luminol under the irradiation of LEDs and generates CL. Taking the advantage of the small size nature of LEDs, the constructed photoreactor is greatly miniaturized, and especially suitable as a CE detector. The feasibility of the proposed detector was evaluated by detection of riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) after CE separation. Under the optimized conditions, the LODs for RF, FMN and FAD were 0.007, 0.02 and 0.1 μg/mL, respectively, better than those by UV detection. The RSDs were 3.4, 3.6 and 4.1% for 0.5 μg/mL RF, 2 μg/mL FMN and 5 μg/mL FAD, respectively. The LED‐CL detector features low cost, miniaturization, fast response, high sensitivity and good reproducibility.     

Light-emitting diode-induced fluorescence detection of native proteins in capillary electrophoresis

A continuous-wave 280 nm light-emitting diode (LED) was used as the excitation source for native fluorescence detection of proteins in CE. The operating current and temperature of the LED were optimized in order to achieve high luminescence power. It was found that a forward current of 30 mA and a temperature of approximately 5 degrees C gave the best S/N. By using a set of two ball lenses to focus light from the LED, we achieved a spot of approximately 200 mum with a power of 0.1-0.2 mW on the detection window. Fluorescence was collected with a ball lens at 90 degrees angle through a bandpass filter onto a photomultiplier tube. In CZE an LOD of 20 nM for conalbumin was reached. In capillary gel electrophoresis all eight proteins from a commercial standard kit were detected with high S/N. For a 10 microg/mL total protein mixture, S/N was better than 3 for all proteins in solution. Further improvement in LOD should be possible on utilization of an LED with higher luminescence power.     

Utilisation of pH stacking in conjunction with a highly absorbing chromophore, 5-aminofluorescein, to improve the sensitivity of capillary electrophoresis for carbohydrate analysis

This study explores the use of pH stacking in conjunction with 5-aminofluorescein as a derivatization agent for the sensitive analysis of simple sugars such as glucose, lactose and maltotriose by capillary electrophoresis (CE). The derivatization agent was selected on the basis of its extremely high molar absorptivity, its compatibility with a 488nm light-emitting diode (LED) and the fact that it has two ionizable groups making it compatible with on-line stacking using a dynamic pH junction. The influence of both acetic and formic acids at concentrations of 0.19, 0.019 and 0.0019molL(-1) were investigated with regard to both derivatization efficiency and the ability to stack using a dynamic pH junction. Superior sensitivity and resolution was obtained in formic acid over acetic acid. Substantially lower peaks were obtained with 0.19molL(-1) formic acid when compared to 0.019 and 0.0019molL(-1) concentrations, which was confirmed by computer simulation studies to be due to the inadequate movement of the pH boundary for stacking. Further simulation studies combined with experimental data showed the separation with the best resolution and greatest sensitivity when the carbohydrates were derivatized with the 0.095molL(-1) formic acid. Utilisation of stacking via dynamic pH junction mode in conjunction with LED detection enabled efficiencies of 150,000 plates and detection limits in the order of 8.5x10(-8)molL(-1) for simple sugars such as glucose, lactose and maltotriose hydrate. The current system also demonstrates a 515 times improvement in sensitivity when compared to using a normal deuterium lamp, and 16 times improvement over other systems using LEDs.     

Novel fused-LEDs devices as optical sensors for colorimetric analysis

The development of a novel, low power optical sensing platform based on light emitting diodes (LEDs) is described. The sensor is constructed from a pair of LEDs fused together at an angle where one LED functions as the light source and the other LED is reverse biased to function as a light detector. Sensor function is based on the level of light received by the detector diode, which varies with the reflectance of the interface between the device and its environment, or the chemochromic membrane that covers the device. A simple microprocessor circuit is used to measure the time taken for the photon-induced current to discharge the detector LED from an initial 5 V (logic 1) to 1.7 V (logic zero). This sensing device has been successfully used for colour and colour-based pH measurements and offers extremely high sensitivity, enabling detection down to the sub micro molar level of dyes.     

Enhancement of signal-to-noise level by synchronized dual wavelength modulation for light emitting diode fluorimetry in a liquid-core-waveguide microfluidic capillary electrophoresis system

The signal-to-noise level of light emitting diode (LED) fluorimetry using a liquid-core-waveguide (LCW)-based microfluidic capillary electrophoresis system was significantly enhanced using a synchronized dual wavelength modulation (SDWM) approach. A blue LED was used as excitation source and a red LED as reference source for background-noise compensation in a microfluidic capillary electrophoresis (CE) system. A Teflon AF-coated silica capillary served as both the separation channel and LCW for light transfer, and blue and red LEDs were used as excitation and reference sources, respectively, both radially illuminating the detection point of the separation channel. The two LEDs were synchronously modulated at the same frequency, but with 180°-phase shift, alternatingly driven by a same constant current source. The LCW transferred the fluorescence emission, as well as the excitation and reference lights that strayed through the optical system to a photomultiplier tube; a lock-in amplifier demodulated the combined signal, significantly reducing its noise level. To test the system, fluorescein isothiocyanate (FITC)-labeled amino acids were separated by capillary electrophoresis and detected by SDWM and single wavelength modulation, respectively. Five-fold improvement in S/N ratio was achieved by dual wavelength modulation, compared with single wavelength modulation; and over 100-fold improvement in S/N ratio was achieved compared with a similar LCW-CE system reported previously using non-modulated LED excitation. A detection limit (S/N = 3) of 10 nM FITC-labeled arginine was obtained in this work. The effects of modulation frequency on S/N level and on the rejection of noise caused by LED-driver current and detector were also studied.     

Performance of an ultraviolet light-emitting diode-induced fluorescence detector in capillary electrophoresis

The performance of a fluorescence detector in capillary electrophoresis (CE) using a light-emitting diode (LED) as excitation source is reported. An ultraviolet LED pulsed at a repetition rate of 500 Hz, combined with a time-discrimination and averaging acquisition system, was used. Limits of detection of 3 and 18 fmoles (at a signal-to-noise ratio equal to 3) were achieved for fluorescamine-derivatized bradykinin and lysine, respectively. This system exhibited a linear response for a concentration range between 54 and 417 microM for derivatized lysine, and between 1.81 and 23.58 microM for derivatized bradykinin. This detection system showed to be very convenient for routine analytical applications.     

Determination of phosphate using a highly sensitive paired emitter-detector diode photometric flow detector

The use of a novel inexpensive photometric device, a paired emitter-detector diode (PEDD) has been applied to the colorimetric determination of phosphate using the malachite green spectrophotometric method. The novel miniaturized flow detector applied within this manifold is a highly sensitive, low cost, miniaturized light emitting diode (LED) based detector. The optical flow cell was constructed from two LEDs, whereby one is the light source and the second is the light detector, with the LED light source forward biased and the LED detector reversed biased. The photocurrent generated by the LED light source discharges the junction capacitance of the detector diode from 5 V (logic 1) to 1.7 V (logic 0) and the time taken for this process to occur is measured using a simple timer circuit.The malachite green (MG) method employed for phosphate determination is based on the formation of a green molybdophosphoric acid complex, the intensity of which is directly related to phosphate concentration. Optimum analytical parameters such as reaction kinetics, reagent to sample concentration ratio and emitter wavelength intensity were investigated for the spectrophotometric method. Linear calibration plots that obeyed the Beer-Lambert law were obtained for phosphate in the range of 0.02-2 μM. The dynamic range, sensitivity and limits of detection are reported.