Individual Pd nanowire hydrogen sensors fabricated by electron-beam lithography.

We report on the hydrogen gas (H2) sensing performance of lithographically patterned Pd nanowires as a function of the nanowire thickness and H2 concentration. A combination of electron-beam lithography and a lift-off process has been utilized to fabricate four-terminal devices based on individual Pd nanowires with width w = 300 nm, length l = 10 microm, and thickness t = 20-400 nm from continuous Pd films. The variation of the resistance and sensitivity at 20 000 ppm H2 of Pd nanowires was found to be much lager than at 10 000 ppm H2, which can be explained by an alpha-beta phase transition occurring at 20 000 ppm H2. This is confirmed by the observation of hysteresis behavior in the resistance versus H2 concentration for Pd thin films. The response time was found to decrease with decreasing thickness regardless of H2 concentration due to a higher surface-to-volume ratio and a higher clamping effect. A single Pd nanowire with t = 100 nm was found to successfully detect H2 at a detection limit of 20 ppm. Our results suggest that lithographically patterned Pd nanowires can be used as hydrogen gas sensors to quantitatively detect H2 over a wide range of concentrations.

Finite size effect on hydrogen gas sensing performance in single Pd nanowires.

We present the hydrogen sensing performance of individual Pd nanowires grown by electrodeposition into nanochannels of anodized aluminum oxide (AAO) templates investigated as a function of the nanowire diameter. Four-terminal devices based on individual Pd nanowires were found to successfully detect hydrogen gas (H(2)). Our experimental results show that the H(2) sensing sensitivity increases and the response time decreases with decreasing diameter of Pd nanowires with d = 400, 200, 80 and 20 nm, due to the high surface-to-volume ratio and short diffusion paths, respectively. This is in qualitatively good agreement with simulated results obtained from a theoretical model based on a combination of the rate equation and diffusion equation.

Comparison between transmittance and reflectance measurements in glucose determination using near infrared spectroscopy.

Glucose determination based on near-IR spectroscopy is investigated for reflectance and transmittance measurement. A wavelength range is 1100 to 2500 nm, which includes both the combination and overtone bands of glucose absorption. Intralipid solutions are used as samples, where glucose concentrations vary between 0 and 1000 mg/dl. Sample thickness for reflectance is 10 cm and 1- and 2-mm-thick samples are used for transmission. Partial least-squares regression (PLSR) analyses are performed to predict glucose concentrations. The standard errors of calibration are comparable between reflectance and 2-mm-thick transmittance. The reflectance method is inferior to the transmittance method in terms of the standard errors of prediction. Loading vector analysis for reflectance does not show glucose absorption features. Reflected light may not have enough information of glucose since a major portion of detected light has a short optical path length. In addition, prediction becomes more dependent on medium scattering rather than glucose, compared with transmission measurement. Loading vectors obtained from a PLSR transmittance analysis have glucose absorption profiles. The 1-mm-thick samples give better results than the 2-mm-thick samples for both calibration and prediction models. The transmittance setup is recommended for noninvasive glucose monitoring.

Development of a compact home health monitor for telemedicine.

A compact and easy-to-use home health monitor was developed. A palm-size health monitor contained a finger probe as sensor unit. In the finger probe, light from a light emitting diode (LED) array was illuminated on a finger nail bed, and transmitted light was measured to obtain photoplethysmography (PPG) signals. Hematocrit, pulse, respiration rate, and saturated oxygen in arterial blood (SpO(2)) were measured simultaneously from PPGs using five different wavelengths: 569, 660, 805, 904, and 975 nm. To predict hematocrit, a dedicated algorithm was used based on scattering theory of red blood cells using these wavelengths. Preliminary clinical tests showed that the achieved percent errors were +/- 8.2% for hematocrit when tested with 549 persons (N = 549). Digital filtering techniques were used to extract respiratory information from a single PPG signal. SpO(2) was predicted on the basis of the ratio of the wavelengths 660 nm and 940 nm. The accuracies were within clinically acceptable errors. In addition, the compact home health monitor included a blood pressure monitoring unit. For convenient and simultaneous measurement with the other previously mentioned signals, blood pressure was measured on a finger. An air cuff was installed on the same finger where PPGs were measured. Achieved mean differences were +/- 3.8 mmHg for systole and +/- 5.1 mmHg for diastole. One can use the palm-size monitor simply by inserting a finger into the home health monitor that is suitable for telemedicine.

Determination of glucose concentration in a scattering medium based on selected wavelengths by use of an overtone absorption band.

A method and device for measuring glucose concentration in a scattering medium have been developed. A spectral range of 800-1800 nm is considered for wavelength selection because of its deeper penetration into biological tissue and the presence of a glucose absorption band. An algorithm based on selected wavelengths is proposed to minimize interference from other components. The optimal distance between the light source and the detector for diffuse reflectance measurement minimizes the influence of medium scattering. The proposed algorithm and measuring device are tested with a solution containing milk with added glucose. Glucose concentrations between 0 and 2000 mg/dl are determined with a correlation coefficient of 0.977. We also investigate the influence of concentration variations of other substances such as water, hemoglobin, albumin, and cholesterol when they are mixed in a scattering medium.

Noninvasive total hemoglobin measurement.

Wavelength selection and prediction algorithm for determining total hemoglobin concentration are investigated. A model based on the difference in optical density induced by the pulsation of the heart beat is developed by taking an approximation of Twersky's theory on the assumption that the variation of blood vessel size is small during arterial pulsing. A device is constructed with a five-wavelength light emitting diode array as the light source. The selected wavelengths are two isobestic points and three in compensation for tissue scattering. Data are collected from 129 outpatients who are randomly grouped as calibration and prediction sets. The ratio of the variations of optical density between systole and diastole at two different wavelengths is used as a variable. We selected several such variables that show high reproducibility among all variables. Multiple linear regression analysis is made in order to predict total hemoglobin concentration. The correlation coefficient is 0.804 and the standard deviation is 0.864 g/dL for the calibration set. The relative percent error and standard deviation of the prediction set are 8.5% and 1.142 g/dL, respectively. We successfully demonstrate the possibility of noninvasive hemoglobin measurement, particularly, using the wavelengths below 1000 nm.