Optical Dissolved Oxygen Sensors Research Articles

Research Progress of Optical Dissolved Oxygen Sensors: A Review

Research Progress of Optical Dissolved Oxygen Sensors: A Review

Calibration method of multi-parameter compensation for optical dissolved oxygen sensor in seawater based on machine learning algorithm

Measurement accuracy of optical dissolved oxygen sensor in actual seawater field is affected by many environmental factors. Traditional sensor calibration method is carried out by combining laboratory calibration experiment with field environmental factor correction. First, the influence of temperature on the sensor measurement results is added to the laboratory calibration experiment and model research. The measurement results are then corrected according to salinity and depth environmental factors in the field measurement. However, the influence of these environmental factors on the dissolved oxygen measurement results presents a multi-parameter nonlinear coupling characteristic. Nonlinear machine learning method is used in this study to investigate coupling mapping relationship between seawater multi-environmental factors and measured data of the sensor. A calibration device for controlling temperature, salinity, hydrostatic pressure, and dissolved oxygen concentration conditions and a matching multi-parameter calibration experimental process is designed and established. Analysis of the coupling relationship between multi-parameter conditions and their influence on the measurement of dissolved oxygen concentration showed that the established multi-parameter compensation calibration model achieves a calibration error of ±1 μmol L−1. Compared with existing compensation results after independent calibration of multi-parameters, the proposed model demonstrates better accuracy and rationality. The calibration sensor is tested in the laboratory and seawater field to verify the validity of the calibration model. Dissolved oxygen concentration measured by the sensor is basically consistent with reference measurement data and Winkler analysis value. Results showed that the multi-parameter calibration method based on machine learning can meet calibration requirements of the dissolved oxygen sensor in seawater with improved scientific rationality and calibration efficiency. Furthermore, the proposed method can be extended to other in situ measurement sensors in seawater while considering various environmental factor corrections.

New method for calibrating optical dissolved oxygen sensors in seawater based on an intelligent learning algorithm.

Oxygen sensors based on luminescence quenching are the most commonly used instruments for in situ measurement in seawater due to their accuracy and long-term stability. The calibration method of the sensor is crucial for their accuracy. Conventional methods exhibit some defects, such as strict control of calibration conditions and cumbersome and time-consuming operation. To improve calibration operation and obtain good calibration results, a new calibration method was proposed for the optical dissolved oxygen sensor in seawater based on an intelligent learning algorithm. The sensor to be calibrated and the reference sensor were deployed in the water for synchronous measurements. The calibration system consisted of a temperature-regulated device and a sampling method to improve calibration operation. An intelligent learning algorithm was used to train the calibration data and model the oxygen response of the sensor. Calibration and test results in both laboratory and field showed that the new calibration method is feasible and efficient. It is highly significant for sensor development and in situ measurement in seawater.

The effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from a native distributional range in Japan

We determined the effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from its native distributional range in Japan by using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–irradiance curves at three temperatures (8, 20, and 28°C) showed that the maximum net photosynthetic rates (NPmax) and saturation irradiance were highest at 28°C. Gross photosynthesis determined at 8–36°C (every four increments) and 300 μmol photons m−2 s−1 showed that the maximum gross photosynthetic rate (GPmax) occurred at 19.5°C ( $$ {T}_{\mathrm{opt}}^{\mathrm{GP}} $$ ), which is consistent with the seawater temperature at its peaked abundance in Japan. The maximum quantum yields (Fv/Fm) of photosystem II (PSII) during the 72-h temperature exposures were above 0.60 at 8–28°C but dropped at higher temperatures. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m−2 s−1 at three temperatures showed remarkable decline in the effective quantum yields (ΔF/Fm') of PSII under high irradiance at 8°C only; the Fv/Fm measured after 12-h dark acclimation also did not recover to initial values, signifying its sensitivity to photoinhibition at 8°C. Furthermore, the alga exhibited tolerance to 2 h of desiccation with 80% of water loss from the thallus, and ΔF/Fm' recovered after 24 h of rehydration in seawater, suggesting potential of photosynthetic recovery of this alga at such low hydration threshold. In conclusion, the adaptation of S. muticum to relatively high irradiance, to broad range of temperature (8–28°C), and to desiccation explains its potentially high invasive capacity.

Dissolved Oxygen Measurement in Seawater and Sensor Calibration Method

The development and key calibration methods of the sensors for the measurement and automatic monitoring of dissolved oxygen in seawater have always been the research focus in the field of Marine monitoring technology. This paper summarizes the measurement methods of dissolved oxygen in seawater, the working principle and advantages and disadvantages of different dissolved oxygen sensors, and the research on calibration methods of mainstream optical dissolved oxygen sensors. Especially for optical dissolved oxygen sensors with better stability, longer service life and stronger anti-interference ability, several laboratory calibration methods which have been developed and applied, as well as in situ calibration methods which are still in the research stage are analyzed.

The effects of temperature and irradiance on the photosynthesis of two heteromorphic life history stages of Saccharina japonica (Laminariales) from Japan

This study examined the effects of temperature and irradiance on photosynthetic characteristics of the macroscopic sporophyte (SPO) and microscopic gametophyte (GAM) stages of a subarctic brown alga, Saccharina japonica var. japonica (Laminariales) from Hokkaido, Japan. In vitro measurements under short- and long-term exposures were carried out by using optical dissolved oxygen sensors and the pulse amplitude modulation (PAM)-chlorophyll fluorometer, respectively. The heteromorphic life history stages of S. japonica showed photosynthetic optima at 23–23.3 °C, derived from the gross photosynthesis–temperature model. Maximum quantum yields (Fv/Fm) of SPO and GAM after 72 h of temperature exposures in the dark were reduced to near zero above 24 °C, indicating PSII inactivation. Such similarity in their temperature characteristics suggests the co-occurrence of both generations in the habitat despite the seasonal growth and reproduction of the species. Net photosynthesis–irradiance experiments in the two life history stages at 8, 16, and 24 °C revealed similarity in their light-saturated photosynthetic rates (NPmax = 3.02–4.41 μg O2 gww−1 min−1, SPO; 2.87–3.73 μg O2 gww−1 min−1, GAM), but saturation irradiances of SPO (Ek = 81–102 μmol photons m−2 s−1) were higher than those of GAM (48–69 μmol photons m−2 s−1). A slight decrease in net photosynthetic rates of GAM above 500 μmol photons m−2 s−1 was likewise observed. This difference may be related to the light regime of their natural habitat, suggesting the low irradiance adaptation of the microscopic stage that settles on rock crevices beneath algal canopies.

Effect of light and temperature on photosynthesis of a cultivated brown alga, Saccharina sculpera (Laminariales), from Japan

ABSTRACT Understanding the physiological responses of kelp to environmental parameters is essential for the conservation of natural kelp beds and increased cultivation yield. We investigated the effects of photosynthetically active radiation (PAR) and temperature on photosynthesis of a subarctic edible brown alga, Saccharina sculpera (Laminariales), using pulse amplitude modulation (PAM)–chlorophyll fluorometry and optical dissolved oxygen sensors. Photosynthesis–PAR experiments at 8 °C, 15 °C, and 22 °C revealed the highest net photosynthetic rates at 22 °C. Values for compensation (Ec) and saturation (Ek) indicated a high affinity for a light environment that reflected the natural vertical habitat of this alga. The photosynthetic response of S. sculpera to temperature indicated that the optimum temperature for oxygenic photosynthesis () was 22.9 °C, which was consistent with the highest seawater temperature in Japanese distribution range for this subarctic species. The alga survived for 168 h at 24 °C in darkness. Continuous 6 h exposure to PAR of 100 µmol (low) and 1000 µmol (high) photons m–2 s–1 at 8 °C, 15 °C, and 22 °C induced chronic photoinhibition in the alga, with greater declines in effective quantum yields of photosystem II (ΦPSII). Failure to recover Fv/Fm after dark acclimation at low and high PAR was observed, except for low PAR at 8 °C. The physiological performance of S. sculpera indicates an affinity for a low light environment. We expect that our results will lead to an improvement in aquaculture techniques through the regulation of cultivation depth.

The effect of elevated CO2 on the production and respiration of a Sargassum thunbergii community: A mesocosm study

SUMMARYApproximately one‐third of anthropogenic carbon dioxide is absorbed into the ocean and causes it to become more acidic. The Intergovernmental Panel on Climate Change (IPCC) suggested that the surface ocean pH, by the year 2100, would drop by a further 0.3 and 0.4 pH units under RCP (Representative Concentration Pathway) 6.0 and 8.5 climate scenarios. The macroalgae communities that consisted of Sargassum thunbergii and naturally attached epibionts were exposed to fluctuations of ambient and manipulated pH (0.3–0.4 units below ambient pH). The production and respiration in S. thunbergii communities were calculated from dissolved oxygen time‐series recorded with optical dissolved oxygen sensors. The pH, irradiance, and dissolved oxygen occurred in parallel with diurnal (day/night) patterns. According to net mesocosm production – photosynthetically active radiation (PAR) model, the saturation and compensation PAR, the mean maximum gross mesocosm production (GMP), and daily mesocosm respiration were higher in the CO2 enrichment, than in the ambient condition, while the mean of photosynthetic coefficient was similar. In conclusion, elevated CO2 stimulated oxygen production and consumption of S. thunbergii communities in the mesocosm. Furthermore, the sensitivity of the GMP of the S. thunbergii community to irradiance was reduced, and achieved maximum production rate at higher PAR. These positive responses to CO2 enrichment suggest that S. thunbergii communities may thrive in under high CO2 conditions.

A newly developed feeder and oxygen measurement system reveals the effects of aging and obesity on the metabolic rate of zebrafish.

Metabolic alterations are relevant for the aging process. Declining metabolic rate with age is a common future of many animals, but it is not well understood how it does so. Here, we used zebrafish as a model for understanding how metabolic changes occur during aging and the interaction between aging and obesity on the metabolic rate. The oxygen consumption rate (OCR) has been used as an index of metabolic processes; however, it is difficult to accurately evaluate OCR with movement being considered because zebrafish need to move freely during the OCR measurement. To measure metabolic rate with high accuracy and efficiency, we developed a method for simultaneously collecting data on sequential oxygen consumption and distance moved by zebrafish using optical dissolved-oxygen sensors and the EthoVision video-tracking system as well as an automatic feeding system for zebrafish whereby obese zebrafish were produced by short-term overfeeding treatment. Using these systems, we examined metabolic changes during aging and overfeeding. First, we used 1- to 22-month-old zebrafish to evaluate changes in metabolism during the aging process. Measurements of body mass and length showed that the growth of the body rarely continued beyond 6 months, at which point zebrafish reach adulthood. Spontaneous swimming activity peaked at approximately 6-10 months and declined thereafter. Metabolic rates at low movement dramatically dropped during the first 4 months and gradually decreased with age after 10 months. These data suggest that metabolic aging becomes evident at approximately 10-14 months and that the metabolic rate (low movement) is useful for the detection of age-related metabolic changes in zebrafish. Second, by short-term overfeeding treatment using the automatic feeding system, we found that overweight is a strong risk factor for the development of metabolic disorders in zebrafish, but there was no interaction between obesity and aging on the metabolic rate. Therefore, our data suggest that the aging-related decline in metabolic-rate may be mostly programmed rather than being affected by energy balance disorder.

Photosynthetic performance of Saccharina angustata (Laminariales, Phaeophyceae) at the southern boundary of subarctic kelp distribution in Japan

ABSTRACTEffects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a subarctic edible brown alga, Saccharina angustata (Laminariaceae, Laminariales), from Hokkaido, Japan, were determined using a pulse amplitude modulation (PAM)–chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–PAR experiments at 8, 16, and 20 °C revealed highest net photosynthetic rates at 16 °C; whereas, the temperature effect on maximum quantum yield (Fv/Fm) showed the highest Fv/Fm at 7.0 to 8.4 °C. These temperatures corresponded to the optimum temperatures for growth of Japanese Saccharina species. The maximum gross photosynthetic rate from gross photosynthesis–temperature curve was at 21.2 °C. Though consistent with the highest seawater temperature in the study site, it is close to the limit for thermal inhibition, because Fv/Fm above 20 °C was compromised after 48–72 h of incubation. Continuous 6-h exposures to PAR of 200 µmol (low) and 1000 µmol (high) photons m−2 s−1 at 8, 16, and 20 °C caused chronic photoinhibition in the alga, with greater declines in effective quantum yields of photosystem II (ΦPSII) and failure of recovery in post-dark acclimation Fv/Fm under high PAR. This physiological performance provides a basis for understanding the persistence of S. angustata near its southern boundary in the western Pacific.

Effect of photosynthetically active radiation and temperature on the photosynthesis of two heteromorphic life history stages of a temperate edible brown alga, Cladosiphon umezakii (Chordariaceae, Ectocarpales), from Japan

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a temperate Japanese edible brown alga, Cladosiphon umezakii (Chordariaceae, Ectocarpales), from Honshu Island, Japan, was determined in the heteromorphic life history stages (macroscopic and microscopic) by using pulse-amplitude modulation (PAM) fluorometry and optical dissolved oxygen sensors. The microscopic stage is well-adapted to relatively low PAR environment, as revealed by its lower maximum net photosynthesis (NPmax), saturation (Ek), and compensation PAR (Ec), and higher value of the initial slope (α) compared to the macroscopic stage. Both stages share similar temperature optima ( $$ {T}_{opt}^{GP} $$ 15.9–25.8 °C for macroscopic stage, 12.6–27.4 °C for microscopic stage), suggesting the possible occurrence of both generations in their habitats throughout the year. While these optimum temperatures are within the range of seawater temperature in the distribution of this species, they are close to the physiological limit of thermal inhibition. Continuous exposures (6 h) to 200 (low) and 1000 (high) μmol photons m−2 s−1 at 8, 16, and 28 °C revealed greater declines in effective quantum yields of photosystem II (ΦPSII) in the microscopic stage, confirming its low PAR adaptation. Low temperature-induced photoinhibition was likewise observed in both stages. Existing farming methods for Japanese Cladosiphon (especially for subtropical Cladosiphon okamuranus from Ryukyu Islands, Japan) may need to be modified to meet the optimum requirements of C. umezakii in the temperate region of Japan.

New Possibilities of the Classical Method: Automated Determination of Primary Production and Destruction of Organic Substance in a Reservoir by the Oxygen Method

A device for the automated determination of the primary production (PP) and destruction (D) of organic matter in a water body by the oxygen method is presented. It consists of light and dark bottles with optical dissolved oxygen sensors (Onset, YSI) and the pump changing water in bottles through a predetermined period (3 h). Results of 3-day measurements performed in the Mozhaisk Reservoir have shown that the device describes the studied processes fairly well, significantly facilitating field research. The gross PP obtained during the experiment constituted about 4 mg O2/(L day), which corresponds to the eutrophic status of the reservoir. The device can be used for studying the functional characteristics of aquatic ecosystems under changing environmental factors such as temperature, illumination, and content of biogenic elements.

Photosynthetic responses to photosynthetically active radiation and temperature including chilling‐light stress on the heteromorphic life history stages of a brown alga, Cladosiphon okamuranus (Chordariaceae) from Ryukyu Islands, Japan

SUMMARYPhotosynthetic responses to temperature and photosynthetically active radiation (PAR) were investigated on the heteromorphic life history stages (macroscopic and microscopic stages) of an edible Japanese brown alga, Cladosiphon okamuranus from the Ryukyu Islands. Measurements were carried out by using optical dissolved oxygen sensors and a pulse‐amplitude modulated fluorometer. Maximum net photosynthetic rates and other parameters of the Photosynthesis – PAR curves at 28°C were somewhat similar in both life history stages, without characteristic photoinhibition at 1000 μmol photons m−2 s−1. Results of oxygenic gross photosynthesis and dark respiration experiments over a temperature range of 8–40°C revealed similar temperature optima for both stages (29.7°C, macroscopic stage; 30.3°C, microscopic stage), which support their observed occurrences in the habitat during summer. Maximum quantum yields of photosystem II (PSII) (F v/F m) were relatively stable at low temperatures with the highest at 15.1°C for the macroscopic stage and at 16.5°C for the microscopic stage; but dropped at higher temperatures especially above 28°C. Continuous exposures (6 h) to 200 and 1000 μmol photons m−2 s−1 at 8, 16, and 28°C revealed greater depressions in effective quantum yields of PSII (Φ PSII) of the microscopic stage at 8°C, as well as its F v/F m that barely increased after 6 h of dark acclimation. Whereas post‐dark acclimation F v/F m of both stages exposed to low PAR fairly recovered at 28°C, suggesting their photosynthetic tolerance to such high temperature. Under natural conditions, both heteromorphic stages of C. okamuranus may persist throughout the year in this region. Beyond its northern limit of distribution, the microscopic stage of this species may suffer from photodamage, as enhanced by low winter temperatures; hence, its restricted occurrence.

Oxygen sensitive polymeric nanocapsules for optical dissolved oxygen sensors

Immobilization of the oxygen-sensitive probes (OSPs) in the host matrix greatly impacts the performance and long-term usage of the optical dissolved oxygen (DO) sensors. In this work, fluorescent dyes, as the OSPs, were encapsulated with a crosslinked fluorinated polymer shell by interfacial confined reversible addition fragmentation chain transfer miniemulsion polymerization to fabricate oxygen sensitive polymeric nanocapsules (NCs). The location of fluorescent dyes and the fluorescent properties of the NCs were fully characterized by fourier transform infrared spectrometer, x-ray photoelectron spectrometer and fluorescent spectrum. Dye-encapsulated capacity can be precisely tuned from 0 to 1.3 wt% without self-quenching of the fluorescent dye. The crosslinked fluorinated polymer shell is not only extremely high gas permeability, but also prevents the fluorescent dyes from leakage in aqueous as well as in various organic solvents, such as ethanol, acetone and tetrahydrofuran (THF). An optical DO sensor based on the oxygen sensitive NCs was fabricated, showing high sensitivity, short response time, full reversibility, and long-term operational stability of online monitoring DO. The sensitivity of the optical DO sensor is 7.02 (the ratio of the response value in fully deoxygenated and saturated oxygenated water) in the range 0.96–14.16 mg l−1 and the response time is about 14.3 s. The sensor’s work curve was fit well using the modified Stern–Volmer equation by two-site model, and its response values are hardly affected by pH ranging from 2 to 12 and keep constant during continuous measurement for 3 months. It is believed that the oxygen sensitive polymeric NCs-based optical DO sensor could be particularly useful in long-term online DO monitoring in both aqueous and organic solvent systems.

トリス（4,7-ジフェニル-1,10-フェナントロリン）ルテニウム（II） を内包したゲル微粒子による光学型溶存酸素センサ

トリス（4,7-ジフェニル-1,10-フェナントロリン）ルテニウム（II） を内包したゲル微粒子による光学型溶存酸素センサ

Green optical dissolved oxygen sensor based on a chlorophyll–zinc complex extracted from the plant Brassica oleracea L

In this study, we present the development of a green and high-sensitivity optical dissolved oxygen (DO) sensor for measuring DO in water using spectrofluorimetry. For the development of the sensor element, an organometallic complex was synthesized from the “chlorophyll A” molecule, extracted from the plant Brassica oleracea L., with zinc insertion (Zn+2) replacing the magnesium (Mg+2). In the investigation, we evaluated the fluorescence suppression of the organometallic zinc complex for samples with different concentrations of DO. The complex has shown two absorption regions, 350–475 nm and 600–700 nm. We observed the fluorescence suppression of the complex, when excited at 440 nm and analyzed at 635 nm, by using the spinning oxygenation and air pump flow methods, respectively. The fluorescence suppression curves presented first-order decay with good correlation, resulting in R2=0.98282 and R2=0.83849 for the spinning and air pump flow methods, respectively. To validate the methodology, we developed a prototype of a bench sensor. For the relation between the fluorescence intensity and oxygenation time, we obtained a ratio curve with R2=0.9809. The methodology and prototype sensor developed in this work are presented as a new optical method for the measurement of DO.

In Situ Benthic Nutrient Flux and Sediment Oxygen Demand in Barnegat Bay, New Jersey

Wilson, T. and DePaul, V., 2017. In situ benthic nutrient flux and sediment oxygen demand in Barnegat Bay, New Jersey. In: Buchanan, G.A.; Belton, T.J., and Paudel, B. (eds.), A Comprehensive Assessment of Barnegat Bay-Little Egg Harbor, New Jersey.The U.S. Geological Survey, in cooperation with the New Jersey Department of Environmental Protection, measured sediment oxygen demand (SOD) and benthic nutrient fluxes throughout Barnegat Bay, New Jersey. SOD was determined in situ using chambers equipped with optical dissolved oxygen sensors. The benthic nutrient fluxes of ammonia (NH3), nitrite nitrate ( plus ions; here, referred to as NO32), soluble reactive phosphorous (SRP), and dissolved silica (SiO2) were measured with in situ equilibrium dialysis samplers. Measurements were made at nine stations around the periphery and at three mid-Bay locations from August 2012 to October 2013. The SOD ranged from −1.5 to −8.4 g of oxygen (O2) m−2 d−1. The SOD rates varied as a function of water temperature and followed the van't Hoff rate equation for change in reaction rate with temperature, with a temperature coefficient (Θ) that varied among sites and averaged 1.083. The highest SOD rates in the bay were measured near the mouth of the Toms River embayment. Concentrations in the upper 1 m of sediment pore water were found up to 23 mg N L−1 for NH4 and 6.7 mg P L−1 for SRP. Maximum measured fluxes into the overlying water were 3.0 × 10−2 g NH3–N m−2 d−1, 7.0 × 10−4 g NO32–N m−2 d−1, 1.9 × 10−3 g P m−2 d−1, and 3.6 × 10−3g SiO2 m−2 d−1. Using the measured benthic N and P fluxes, daily nutrient inputs derived from sediment recycling are shown to be comparable in scale to freshwater tributary inputs to the bay.

The primary production of Amursky Bay (Sea of Japan) in the summer of 2008

In July 2015, the assimilation number (An) of phytoplankton in Amursky Bay was measured using optical dissolved oxygen sensors (Rinko). The primary production (PP) in the photic layer of Amursky Bay was calculated based on the measured An and chlorophyll vertical profiles obtained during a hydrochemical survey on August 23−28, 2008. The total production in the bay at that period was 840 tC/day. During a phytoplankton bloom, the excess production of biomass leading to the hypoxia of the bottom water was estimated to be approximately (1.0−2.0) × 103 tC/day.

(Invited) A Frequency Domain Optofluidics Dissolved Oxygen Sensor with Enhanced Sensitivity for Water Monitoring

Dissolved oxygen (DO) is an important indicator for water quality and is also used in point of care applications. Fluorescence signal from Ruthenium (Ru(dpp)3 2+ ) based fluorophores can be quenched by oxygen such that the reduction of fluorescence signal is proportionally to oxygen concentration. This mechanism allows an optical method to measure DO in liquid. Although optical DO measurements offer high sensitivity and stability, they are also expensive, bulky, and difficult to use due to its complex design and requirement for specialized optical instruments such as light sources and spectrometers. Recently, the advances in microfabricated optical component including waveguides, lenses and mirrors, light sources, and photo detectors enable the integration of optical sensing and microfluidics sampling platform. We report the development of a total internal reflection assisted (TIRA) optical DO sensor for sensitivity enhancement. The excitation light is conducted down the water channel direction in the microfluidic device, while being confined within glass slide by total internal reflection. In addition to steady state intensity measurements, oxygen quenching is measured in the frequency domain using modulated light sources and synchronized detection systems. Experimental results show that optical sensitivity can be increased up to an order of magnitude in TIRA sensors. These results suggest the potential of TIRA scheme as a sensing and characterizing platform for optofluidic sensors.

Development of a Miniaturized Dissolved Oxygen Sensor with Anti-Biofouling Coating for Water Monitoring

The consumption of dissolved oxygen (DO) in water is indicative of aqueous organic content and therefore contamination in water1. Optical DO sensors consist of an oxygen sensitive fluorescence membrane on which oxygen can cause the quenching effect; the quenched fluorescence intensity is proportional to oxygen concentration2. These optical DO sensors are highly sensitive and long-term stable; however, the complex optical design and optical components make the cost always more expensive than electrochemical DO sensors. In order to reduce the cost, microfabricated optical DO sensing device is developed3; nevertheless, the sensitivity is sacrificed owing to the small sensing area resulting from miniaturization. Conventional optical DO sensing setup has single signal excitation site which contributes to the fact that the devices cannot provide sufficient signal intensity when being miniaturized (Fig. 1 (a)). In the light of this, we designed a multi-reflection (MR) optical DO sensor for overall sensitivity enhancement. Experimental results showed that the sensitivities can be increased by more than 3 times in MR sensors. In another aspect, biofouling is one of the major challenges of all water sensors4; therefore, we grafted polyethylene glycol (PEG) on the raw materials (polydimethylsiloxane, PDMS) of DO sensitive membrane. Reduction of protein adsorption (first step of biofouling) was observed as a result. Experiments Results and Discussions The optical scheme of MR optical DO sensor is shown in Fig. 1 (b). The fluorescence DO sensitive membrane was immobilized in a reservoir with total reflective (gold) surface at its bottom. A 455 nm laser was used as light source for generating fluorescence. A fluorescence-choosing band-pass filter is placed on top of the reservoir for not only preventing the excitation light from detected by a CCD sensor but also reflecting the excitation light into the reservoir again for MR purpose. After excitation, the generated fluorescence from membrane was collected by a convex lens and read by a commercially available spectrometer. The synthesis of DO sensitive membrane is guided by the following procedure: first, the PDMS elastomer (Dow Corning) was mixed with crosslinker at 10 to 1 weight ratio. Then 1 mg of the luminescent dye (Tris(bipyridine)ruthenium(II) chloride, Sigma Aldrich) was added to the polymer solution and stirred until uniformly mixed. The mixture was spin-coated on a three-inch silicon wafer at 4000 rpm for 30 second to obtain a membrane with 20 µm thickness. The DO sensing results suggest that the sensitivity is increased three times when using MR device as compared with the conventional setup.Contact angles of PDMS surfaces before and after PEG grafting are shown in Fig. 2 (a). The results show that the contact angle of PDMS is reduced significantly after PEG grafting (from 106.4 to 24.3), which illustrates the facts that the hydrophobicity of PDMS is reduced and that the protein (hydrophobic) absorption is restricted as showed in Fig. 2 (b). Conclusion The MR DO sensing device shows three times higher sensitivity as compared with signal excitation optical DO sensing setup. The result suggests that the multi-reflection device is a good replacement of miniaturized optical DO sensor. PEG grafting results in our experiments also show good anti-protein absorption capacity which may be effective in eliminating the biofouling progress. Reference 1. U.S. Environmental Protection Agency. in Volunteer Estuary Monitoring: A Methods Manual(U.S. Environmental Protection Agency) 9–3 (Office of Water, Washington, DC, 2006).2. McDonagh, C., Maccraith, B. D. & McEvoy, a K. Tailoring of sol-gel films for optical sensing of oxygen in gas and aqueous phase. Analytical chemistry 70,45–50 (1998).3. McEvoy, A., McDonagh, C. & MacCraith, B. Dissolved oxygen sensor based on fluorescence quenching of oxygen-sensitive ruthenium complexes immobilized in sol–gel-derived porous silica coatings. Analyst 121,785–788 (1996).4. Flemming, H.-C. Biofouling in water systems--cases, causes and countermeasures. Applied microbiology and biotechnology 59, 629–40 (2002).