In Vivo Monitoring of Oxygen in Rat Brain by Carbon Fiber

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In Vivo Monitoring of Oxygen in Rat Brain by Carbon Fiber Microelectrode Modified with Anti-fouling Nanoporous Membrane Lin Zhou, Hanfeng Hou, Huan Wei, Lina Yao, Lei Sun, Ping Yu, Bin Su, and Lanqun Mao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05658 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Analytical Chemistry

In Vivo Monitoring of Oxygen in Rat Brain by Carbon Fiber Microelectrode Modified with Anti-fouling Nanoporous Membrane Lin Zhou,a Hanfeng Hou,b Huan Wei,b Lina Yao,a Lei Sun,a Ping Yu,b Bin Su,*a and Lanqun Mao*b a

Institute of Analytical Chemistry, Department of Chemistry, Zhejiang University, Hangzhou 310058, China. Beijing National Research Center for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China. b

ABSTRACT: Oxygen (O2) is involved in many life activities and its in vivo monitoring is of vital importance. In vivo electrochemistry with carbon fiber microelectrode (CFME) has been proved to be a suitable technique, but the surface fouling propensity poses a great challenge to its current stability and reliability. Herein we electro-grafted silica nanoporous membrane (SNM) consisting of uniform, closely packed and vertically aligned nanochannels on the CFME surface, which was capable of protecting the surface effectively from biofouling and, meanwhile, preserving the permeability to O2. In comparison with a bare CFME, the SNM/CFME after implantation in the brain of live rat maintained its analytical sensitivity to O2. Moreover, the implanted electrode could monitor O 2 continuously under the in vivo condition, exhibiting an excellent current stability, as well as a fast response, up to 2 h. Further considering the high permeability, selectivity and biocompatibility of SNM, we believe the modified CFME is a highly reliable sensor for long-term in vivo monitoring of O 2, as well as other neurochemicals, with promise in physiological, ethological and neurological studies.

O2 is an essential element in organisms and involved in numerous neurochemical processes. Neural injury, brain dysfunction and many other diseases are closely associated with abnormal levels of O2. For example, brain ischemia, diabetes and cancer will result in hypoxia. While a high level of O2 in tissues will lead to O2 toxicity and neuropsychiatric disorder.1,2 Therefore, in vivo monitoring of O2 (especially brain O2) is of vital significance. Many electrochemical techniques have been developed for this purpose,3-9 amongst which platinum and carbon fiber microelectrode (CFME) behave well.10-19 However, due to surface fouling arising from the nonspecific adsorption of unwanted chemical substances, biological molecules and microorganisms, the microelectrodes often suffer from current instability and gradual loss of sensitivity particularly after a long period of usage. Although large sized microelectrodes (a few hundreds of micrometer in diameter) have been employed to monitor O2 in vivo successfully with a satisfactory long-term stability,20-24 they may induce irreversible tissue injure when being implanted in rat brain.25 So small sized microelectrodes, such as CFME with a diameter of 7 μm, have been used in recent years, with additional advantages of high sensitivity and fast response. However, they are susceptible to more sever surface fouling because of their small surface area. So preventing the electrode surface from biofouling remains to be an important issue for in vivo monitoring of O2.2628 On the other hand, the electrode implantation can also induce foreign-body response due to innate immunity, resulting in the fibrotic encapsulation of electrode dependent on its biocompatibility.29,30 Both surface fouling and foreign-body response can block electrochemically active surface of electrodes, leading to low sensitivity, high limit of detection (LOD), long response time, slow charge and mass transfer kinetics.31,32 The accuracy and reproducibility of analysis are thus unreliable. As a result, improving anti-fouling capability and biocompatibility of electrode surface is highly important. One of conventional antifouling approaches is to coat or modify the surface with a broad

range of materials, including polymers, peptides, proteins, zwitterions, self-assembled monolayers and porous structures,33-39 which have been demonstrated to work effectively for in vitro analysis and in many complex samples, such as serum, urine and even blood. However, additional factors must be taken into account for in vivo analysis, such as stability, toxicity and aforementioned biocompatibility.40 Very recently, bovine serum albumin (BSA), polyethylenedioxythiophene/phosphoryl-choline, nafion, base-hydrolyzed cellulose acetate (BCA), polyethylenedioxythiophene/Nafion, fibronectin, carbon nanotubes (CNTs) and polyurethane have been employed to tailor the microelectrode surface for in vivo analysis.26,41-49 Although these materials are against fouling, low toxic and highly biocompatible, they are susceptible to biological and chemical degradation upon using for a period of time and thus unsuited to long-term in vivo applications for studying many time-dependent physiological processes and ethology of freely moving animals.34,50-54 So anti-fouling materials with enhanced stability, low toxicity and high biocompatibility remain to be highly demanded.21.55 Among various anti-fouling materials, porous inorganic structures have received considerable attentions over the past few years,37,56-70 because of their high mechanical strength, chemical stability and biocompatibility. For example, porous gold and silica nanochannel membrane (SNM) modified electrodes have been employed for electroanalysis in diverse complex samples, encompassing food, soil, water, biofluids (serum, urine, whole blood) and so on. 56-63 The SNM is particularly interesting because of its additional molecular sieving ability and permeability arising from uniform channel size, vertical orientation and high porosity. It should be noted that, different from in vitro systems (blood fluid, water, food), the living system is more complex. The electrode suffered from severe biofouling and foreign body response. Therefore, it is more difficult to achieve enough antifouling performance under in vivo system. In this work, we attempt to fabricate 1

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SNM-modified carbon fiber microelectrodes (CFMEs) and explore their anti-fouling capability and stability under the in vivo condition. As exemplified in Scheme 1, the nonconductive and highly porous SNM is expected to protect the electrode surface from contacted fouling by biomacromolecules and organisms while preserve the permeability to O 2. The SNM/CFME will be implanted to rat brain for in vivo monitoring of O 2.

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under nitrogen stream. As-prepared SNM/CFME was aged at 100 oC overnight. The CTAB surfactant molecules inside SNM were removed by immersing the electrodes in 0.1 M HCl ethanol solution for 15 min. Structure Characterization of SNM. Scanning electron microscopy (SEM) was conducted on a field emission scanning electron microscopy (SU-8020, Hitachi, Japan). Transmission electron microscopic (TEM) photograph was taken on HT7700 transmission electron microscope (Hitachi, Japan). The TEM specimen was prepared by mechanically peeling off SNM grown on the large-area disk-shaped glassy carbon electrode (3 mm in diameter), dispersing in ethanol under sonication and dropping onto copper grid. Synthesis of FITC-labeled BSA (FITC-BSA) and Fluorescence Measurements. FITC-BSA was prepared as previously reported.72 Briefly, 7.5 mg of FITC was dissolved in 3.75 mL of DMSO, which was then slowly added to BSA aqueous solution (40 mg/mL, pH 7.4) under stirring. The mixture was incubated in dark at 4 oC for 8 h to get an orange solution. Finally, 5 mM of NH4Cl was added to quench the reaction. The prepared FITC-BSA was stored at 4 oC in dark. The fluorescence imaging experiments were carried out on an upright optical microscope (Nikon eclipse LV100ND). A high pressure mercury lamp (Nikon C-SHG1) was used as light source. The electrodes were directly immersed in FITC-BSA in dark for 2 h, washed with deionized water to remove loosely bound FITC-BSA and finally dried under pure N2 for fluorescence imaging. Electrochemistry Measurements. All electrochemical experiments were performed on an electrochemical workstation (CHI 920, Chenhua, Shanghai). A three electrode system was employed, using SNM/CFME, a Pt wire and an Ag/AgCl (saturated KCl solution) as the working, counter and reference electrodes, respectively. For in vivo electrochemical experiments, a tissue-implantable micro-Ag/AgCl was employed as the reference electrode (aCSF was used as the inner solution). In Vivo Experiments. Adult male Sprague-Dawley rats (300350 g) were purchased from Health Science Center, Peking University. The animals were housed on a 12 h:12 h light-dark schedule with food and water ad libitum. All animal procedures

Scheme 1. Illustration of hydrophilic, highly permeable and antibiofouling silica nanoporous membrane (SNM) coated CFME for continuous monitoring of O 2 in rat brain.

EXPERIMENTAL SECTION Chemicals and Reagents. All chemicals and reagents were analytical grade or higher and used as received without further purification. Cetyltrimethylammonium bromide (CTAB, 98%), tetraethoxysilane (TEOS, 99.0%) and hexaammineruthenium (III) chloride (Ru(NH3)6Cl3, 98%) were purchased from Aldrich. Fluorescein 6-Isothiocyanate (isomer II) (FITC) and bovine serum albumin (BSA) were purchased from Aladdin. All aqueous solutions were prepared with ultrapure water (18.2 MΩ cm, MilliQ, Millipore). Carbon fiber (d = 7 μm) was brought from Toray. Artificial cerebrospinal fluid (aCSF) was prepared by adding NaCl (126 mmol), KCl (2.4 mmol), KH2PO4 (0.5 mmol), NaHCO3 (27.5 mmol), Na2SO4 (0.5 mmol), MgCl2 (0.85 mmol) and CaCl2 (1.1 mmol) into 1.0 L ultrapure water. The pH of aCSF was adjusted to 7.2-7.4 with concentrated HCl. The aCSF was stored at 4 oC for use. Electro-grafting SNM on CFMEs. Disk-shaped carbon fiber electrode (d-CFME, d = 7 μm) and column-shaped carbon fiber electrode (c-CFME, d = 7 μm, l = 300 μm) were both used in this work. SNM modified CFMEs were prepared using electroassisted self-assembly (EASA) approach as previously reported.71 Typically, a precursor solution was firstly prepared by mixing ethanol and aqueous NaNO3 (0.1 M) in a volume ratio of 1/1 (v/v), followed by adjusting the pH to 3.0 using concentrated HCl. Subsequently, 109 mM of CTAB and 340 mM of TEOS were immediately added, and the resulting mixture was further stirred for 2.5 h at room temperature. SNM was electro-grafted on dCFME and c-CFME using chronopotentiometry by applying a current density of −3.69 mA cm−2 and 8.0 mA cm2 for 15 s (stainless steel plate and siliver/silver chloride electrode were used as the counter and reference electrodes, respectively). After electro-grafting, SNM/CFMEs were immiediately washed by ultrapure water to remove excess precursor solution and dried

were approved by the Animal Care and Use Committee at National Center for Nanoscience and Technology of China and performed according to their guidelines. Firstly, rats were

anaesthetized with isoflurane (4% induction, 2% maintenance) through a R520 gas pump (RWD, China) and positioned onto a stereotaxic apparatus via the ear rods during in vivo experiments. The SNM/CFME were implanted into hippocampus (AP = 5 mm, L = 5 mm from bregma, V = 4.5 mm from dura), using standard stereotaxic procedures. The reference micro-Ag/AgCl electrode and a Pt wire were placed into the dura of brain. The same volume (about 420 cm3) of pure O2 was given around rat nose for 7 s (note that the short-term exposure to pure O2 would not bring damage to rat brain). The in vivo current response to O2 was recorded by amperometry upon applying a potential of 0.6 V at the microelectrode.

RESULTS AND DISCUSSION The SNM was grown on the CFME surface via the electroassisted self-assembled method.71 Disk-shaped CFME (d = 7 m), designated as d-CFME, was used for electrochemical characterization because its surface area can be precisely controlled. As-prepared SNM/d-CFME was firstly characterized by cyclic voltammetry (CV), using Ru(NH3)63+ and Fe(CN)63 as redox probes. Due to the negatively charged surface arising from 2


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Analytical Chemistry deprotonation of silanol groups and the small channel size (2.3 nm in diameter), the SNM displayed an obvious permselectivity, favoring the transport of counter-ions (namely cations) and inhibiting that of co-ions (namely anions). Therefore, the current magnitude of Ru(NH3)63+ at the SNM/d-CFME was comparable to a bare CFME (see Figure S1a), suggesting the high permeability of SNM. In contrast, that of Fe(CN)63 was highly dependent on the ionic strength. A higher ionic strength led to a larger current (see Figure S1b). In comparison, d-CFME barely responded to the ionic strength (see Figure S1c). These results prove the successful electro-grafting of SNM onto d-CFME. Moreover, the current response of SNM/d-CFME toward Fe(CN)63 in a low ionic strength solution did not vary significantly after 5 h (see Figure S1d), indicating the SNM was compact and remained pretty stable. Because the surface area of CFME is pretty limited, transmission electron microscopy (TEM) characterization of SNM grown on it is pretty hard. So large-area SNM was grown on the glassy carbon electrode (GCE) surface using the same method and subsequently scraped for structure and morphology characterization by TEM. As shown in Figure 1a, silica nanopores that appear as bright dots are highly ordered and closely packed, with a uniform diameter of ca. 2.3 nm. The pore density is about 8.4  1012 cm2, corresponding to a membrane porosity of 34.9%. The crosssection view of TEM photograph shown in Figure 1b reveals that the SNM consists of regularly oriented, parallel channels with a thickness of roughly 79 nm. All these features are similar to those reported previously, 73,74 proving the successful growth of SNM.

Figure 1c shows CVs of SNM/d-CFME and d-CFME in N2, air and O 2 saturated artificial cerebrospinal fluid (aCSF) in the potential range from 0 V to 1.4 V. The faradaic current observed at negative potentials corresponds to the reduction of dissolved O 2. First, the current magnitude displayed by SNM/d-CFME is apparently larger than that at d-CFME. Considering the effective electroactive surface area decreased by roughly 65% after SNM modification, it suggests that the SNM is highly permeable to O 2 and favors the O 2 reduction process. The favorable transport of proton that also participates the reduction reaction of O 2 through nanochannels with silanol groups might also occurs. Fast hopping transport through silica nanochannels has been indeed observed.75,76 Additionally, the reduction of O 2 maybe catalyzed at the ultrasmall surface confined by silica nanopores. Second, in all cases, CVs obtained at SNM/dCFME are characterized by a smaller capacitive charging current, which potentially provides a low background and thus a higher signal-to-noise ratio. Third, the current at SNM/d-CFME was very stable with only ca. 10% of attenuation after 1600 s (see Figure S2), while that at dCFME decreased by about 65%, suggesting the improved current stability of SNM/d-CFME. We subsequently examined the anti-biofouling ability and the current stability of SNM/d-CFME in aCSF containing BSA (10 mg mL1), a commonly used protein in antibiofouling study. The samples were saturated by O 2, in which d-CFME and SNM/d-CFME were immersed to record CVs every 5 min for 60 min. For each CV, the ratio between background-substracted current at 0.6 V at different times (It) and that at t = 0 (I0), namely It/I0 defined as the relative current stability, was plotted against the time. As can be seen from Figure 1d, It/I0 decreased sharply for d-CFME and reached a steady-state after 30 min, with a significant current decrease of about 45%, due to severe surface fouling by BSA. In contast, the current decrease at SNM/d-CFME was much less. Although it continuously decreased, the current magnitude remained to be 85% of its initial value after 60 min. The enhanced current stability can be ascribed to the nanoporous structure of SNM that consists of uniform and ultrasmall channels. The channels (2.3 nm in diameter) are apparently smaller than BSA (MW = 67 kDa, 5.0  7.0  7.0 nm),62,77 so they can effectively protect underlying CFME surface from bio-fouling via the size-exclusion effect. On the other hand, the SNM surface is more hydrophilic (see water contact angle data in Figure S3, supporting information) and negatively charged, which also contributes to the antibiofouling ability. Indeed, as further proved by fluorescence microscopy measurements, the amount of fluorescein isothiocyanate (FITC) labeled BSA adsorbed onto the surface of SNM coated GCE was much lower than that onto bare GCE (see Figure S4, supporting information). Moreover, as shown in Figure S5, the abolute current magnitude of SNM/d-CFME displayed in aCSF containing BSA was large enough (several nanoamper), suggesting the high permeability toward O 2 was preserved. Note that both anti-biofouling ability and permeability are essential to achieve the sensibility toward O 2. The permeability here refers to the accessibility of O 2 to the electrode surface. Numerous surface modification strategies can effectively improve the anti-biofouling ability but at the cost of permeability, because the surface tethered modification layer introduce a high impedance, hindering the mass transport and/or interfacial electron transfer. This issue

Figure 1. (a, b) Transmission electron microscopy (TEM) images showing the top surface (a) and cross-section (b) of SNM with scale bars annotated in the images. The specimen was prepared by scraping the SNM grown on the surface of glassy carbon electrode. (c) Cyclic voltammograms (CVs) obtained with SNM/d-CFME (solid curves) and d-CFME (dashed curves) in artificial cerebrospinal fluid (aCSF) saturated by N2 (black), air (red) and O2 (blue). The scan rate was 100 mV s1. (d) The relative current stability (defined as I/I0) obtained with SNM/d-CFME (red) and d-CFME (black) in O2-saturated aCSF containing 10 mg mL1 of BSA.

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was further rationalized by electrochemical impedance spectroscopy (EIS). EIS of bare GCE and SNM/GCE were comparatively measured in aCSF and aCSF containing BSA (10 mg mL1). As shown in Figure S6, in the case of bare GCE, the presence of BSA led to sharp increase of charge transfer resistance, Warburg impedance and capacitance, due to sever surface biofouling that not only hinders the mass transport but also passivates and deactivates the surface activity. While under the same condition, no obvious change of EIS was observed with SNM/GCE, indicating that both charge transfer and mass transport were not significantly influenced. Although the protein adsorption on the exterior surface SNM cannot be absolutely avoided, the hydrophilic and highly porous structure of SNM largely minimizes the effect on mass transport. Above results are indicative of a high promise for SNM to be used in continuous monitoring of O 2 under the in vivo condition. The in vivo study was performed with SNM/cCFME. It was implanted into the brain of live rat for 2 h and then moved out to measure in-vitro its electrochemical response to O 2 in aCSF. As reported previously, surface biofouling of bare CFME during in vivo implantation results in a significant loss of analytical sensitivity that can be as high as 70%.26 As shown in Figure 2a, the current response due to O2 reduction, as well as the sensitivity defined as the slope of current response versus the concentration (usually called as the post-calibration curve), was maintained at SNM/cCFME after in vivo implantation, although the current magnitude decreased due to the possible diffusion hinderance by fouling species adsorbed on the exterior surface of SNM (as exemplified in Scheme 1). As displayed in Figure 2b, the ratio between sensitivity obtained from post-calibration curve (Spost) and that from pre-calibration (Spre) was estimated to be 0.85  0.04 for SNM/c-CFME. It proved the enhanced current stability by anti-biofouling SNM and the reliability of the modified electrode for in vivo measurements. In comparison, the ratio was decreased to 0.38  0.06 for bare c-CFME, suggesting a large sensitivity loss due to severe surface biofouling.

on the surface of c-CFME (SNM was broken during the SEM sample preparation and thus its boundary could be seen). After in-vivo implantation for 2 h, the surface of SNM/cCFME remained smooth with protein film on it (see Figure 3b). In contrast, as compared in Figure 3c and d, large amount of protein blocks and aggregates were observed for implanted c-CFME. The SEM images corroborate the results of FITC-BSA (see Figure S4).

Figure 3. SEM images of SNM/c-CFME (a, b) and c-CFME (c, d) before (a, c) and after (b, d) in vivo implantation for 2 h.

Figure 2. (a) Pre- and post-calibration curves obtained for SNM/cCFME upon successive increasing the concentration of O2 in aCSF before (red) and after (blue) in vivo implantation in rat brain hippocampus for 2 h. (b) The ratio of sensitivities, Spost/Spre, between post-calibration of SNM/c-CFME (orange column) and c-CFME (red column) after in vivo implantation for 2 h and pre-calibration before implantation in aCSF.

Figure 4. (a, b) Intermittent in vivo monitoring of O2 by SNM/cCFME (a) and c-CFME (b) implanted in the hippocampus of rat brain for 2 h. The rat was fed with the excess pure O 2 for 7 s every 30 min and the amperometric response of implanted electrode was recorded concurrently. The electrode potential was biased at 0.6 V. (c) The amperometric response of implanted SNM/c-CFME to the feeding of N2. (d) The variation of normalized current (I/I0) with time for SNM/c-CFME (red) and c-CFME (blue) during the implantation for 2 h.

The morphology of SNM/c-CFME and c-CFME before and after in vivo implantation was also compared by recording the scanning electron microscopy (SEM) images. As shown in Figure 3a, smooth SNM film could be observed 4


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Analytical Chemistry The superior current stability and anti-biofouling ability allowed us to perform in vivo monitoring of O 2 in rat brain. The rat was fed with the excess pure O 2 for 7 s every 30 min and the amperometric response of implanted electrode biased at 0.6 V was concurrently recorded. The accurate concentration of O 2 in brain was difficult to calculate because of the complex brain condition. However, the amperometric response could be obviously observed while the excess pure O2 was fed. Thus, the current remaining ratio was calculated to characterize the antifouling performance of implanted electrode in vivo. As shown in Figure 4a, SNM/c-CFME always displayed a fast current response to the breath of O 2. First, the response time is comparable to bare c-CFME (see Figures 4b and S7), indicating the coated SNM is highly permeable to O 2. To prove the current response was produced by increasing the level of brain O 2, the rat was fed with pure N 2 (pure N 2 led to the decrease of brain O 2 level), an obvious current decrease was observed (see Figure 4c). It thus confirmed that the recorded current responses in Figure 4a were indeed resulted from breathed O 2. We also found that the electrode was insensitive to potential electroactive interferents, such as hydrogen peroxide, ascorbic acid, uric acid and dopamine, proving the selectivity of SNM/c-CFME toward O 2 (see Figure S8). Note that an increase of noise level was observed for implanted SNM/cCFME (see Figure 4a), which cannot be fully rationalized for the moment and is most likely associated with the fluctuation of normal brain oxygen concentration. Second, as exhibited in Figure 4d, the current response of implanted SNM/c-CFME was pretty stable up to 2 h, with a decrease of only 10%. While under the same condition the current decrease was nearly 80% for c-CFME. As shown in Figure S9, the ratio between sensitivity obtained from postcalibration curve (Spost) and that from pre-calibration (Spre) was estimated to be 0.82  0.02 and 0.25  0.03 for SNM/cCFME and c-CFME, respectively. The results proved the excellent anti-biofouling ability of SNM. In addition, the SNM/c-CFME.

On the other hand, future work will be also directed to the exploration of biocompatibility and foreign-body effect of SNM, although silica is known to be. We also believe that the SNM can also function as a highly porous nanopattern to direct further modification of its exterior surface with another molecular layer, such as PEG, OEG or zwitterions, thus creating more effective anti-biofouling yet electrochemically active interfaces. In this case, the modified CFMEs might work more reliably and operate during extended times, with promise in physiological, ethological and neurological studies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Electrochemical characterization of SNM/CFME, amperometric response, water contact angle measurements, fluorescence tests and electrochemical anti-fouling performances of SNM/CFME are displayed in supporting information.

AUTHOR INFORMATION Corresponding Author Bin Su: [email protected]. Lanqun Mao: [email protected]. ORCID Bin Su: 0000-0003-0115-2279 Lanqun Mao: 0000-0001-8286-9321 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The financial support by the Nature Science Foundation of China (21335001, 21575126 and 21874117) and the Nature Science Foundation of Zhejiang Province (LZ18B050001) is gratefully acknowledged.

CONCLUSION In summary, we report a high anti-fouling CFMEs modified with anti-fouling SNM that consists of ultrasmall, uniform, closely packed and vertically aligned channels. The surface hydrophilicity, high porosity and size-exclusion effect were rationalized to account for the excellent anti-biofouling ability of SNM. Under both in vitro and in vivo conditions, the SNM was capable of preventing effectively the underlying electrode surface from sever biofouling and meanwhile preserving high permeability to O 2. So the SNM/CFME implanted in rat brain worked well for continuous monitoring of O2 up to 2 h, maintaining its analytical sensitivity and fast response. The simple brain oxygen model was employed to investigate the antifouling performances of SNM/CFMEs. Although non-invasive techniques, such as MRI, are also able to detect or even map oxygen in brain, the current approach can be potentially extended to detect a wide range of eletro-active species (such as dopamine, uric acid, ascorbic acid and ROS). In comparison with other antifouling materials, the apparent advantage of SNM is its high permeability toward small molecules, due to its uniform channel size, vertical orientation, high porosity and ultrasmall thickness. A high permeability favors the mass transport of target analytes and thus the analytical sensitivity.

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Analytical Chemistry

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In Vivo Monitoring of Oxygen in Rat Brain by Carbon Fiber

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