Abstract: Introduction Light-addressable potentiometric sensor (LAPS) is a field-effect chemical sensor with the ability of spatial resolution proposed by Hafeman [1] and has an electrolyte-insulator-semiconductor (EIS) structure. With the illumination of focused light, photocurrent can be generated locally, providing the information of surface potential. As a member of potentiometric sensors, technologies for ion-selective electrodes (ISEs) and ion-sensitive field-effect transistor (ISFET) are also applicable to LAPS. For example, by depositing the ion-sensitive membrane (ISM) on the surface of the LAPS, an ion-sensitive LAPS (ISLAPS) can be obtained. Conventional ISEs which contains liquid contacts have some limitations such as requiring maintenance, proper handling, leakage of inner filling solution and primary ion diffusion, which will deteriorate limit of detection. In this work, a multiplex ISLAPS detection system has been proposed combined with Na + , K + , and Ca 2+ all-solid-state ISM and a conventional pH-LAPS. The matrix of the plasticizer free ISM is the silicone-rubber, which has not only the same sensitivity of PVC-based ISM, but also better adhesion and longer lifetime. With the help of a program-controlled two-axis translation stage, the detection sites of the sensor were sequentially illuminated by modulated light from the backside. Different from the conventional multi-channel potential sensors, LAPS system essentially uses a single-channel instrument to realize multi-parameter detection, and can be expanded easily by adding other ISMs only. The multiplex ISLAPS can meet the ion detection requirements, and it is a promising physiology detection platform. Method Figure 1a shows the structure of LAPS chip, the fabrication is similar to the previous report [2], except the thinned illumination area in the center of backside. The sensing material on the oxide layer is 30 nm Al 2 O 3 layer deposited by atomic layer deposition (ALD) [3] for pH detection, and silicone-rubber ISMs for Na + , K + , and Ca 2+ detection. The chip was cut into 1 cm×1 cm with a 4 mm×4 mm thinned area, in turn washed by acetone, ethanol and deionized water to store and use. Preparation of the ISM is as follows: Firstly, about 300mg silicone-rubber (RTV 730) was evenly dissolved in 1.5mL THF and centrifuged. Then the supernatant was mixed evenly with the ionophores and ion additives in 30min ultrasonic bath. Before deposition of ISM, a P3OT layer was applied on the surface of SiO 2 -LAPS chip. After that the ISM mixture was spin-coated on the P3OT layer and dried overnight at room temperature. The three ISM-coated LAPS chips and a Al 2 O 3 -LAPS chip were placed in a PMMA chamber, pasted with conductive silver glue on one bonding pad of PCB, with four holes to expose the thinned area of each site. The edges of chips were encapsulated by epoxy adhesive. The schematic of the multiplex ISLAPS system is shown in Figure 1b. The modulated light with collimator was fixed on the translation stage, illuminating the thinned area of sensor. The data acquisition (DAQ) device performed bias voltage output, signal acquisition, and was controlled by LabVIEW software together with the translation stage. The ISMs are sequentially illuminated and the response of four ions can be obtained in one measurement. Results and Conclusions Sensitivities of the multiplex ISLAPS were calibrated with series of concentration gradient solutions. The background electrolyte for Na + , K + and Ca 2+ is 0.1M CH 3 COOLi, and PBS solutions adjusted with HCl/NaCl were used in pH sensitivity determination. The ISMs were conditioned in the solutions of 10 -2 M corresponding ions for 1h. The I-V curves and the bias-concentration fit lines are shown in Figure 1c. The sensitivity was obtained from the maximum slope points of I-V curves. The limits of detection (LODs) of the Na + , K + and Ca 2+ were about 10 -6 M, and the sensitivities were 57.37mV/pH, 56.9mV/pNa, 58.4mV/pK, and 25.3mV/pCa respectively, close to the Nernst theoretical value. The LOD and linear ranges can meet the requirements of physiological ions detection. The real DMEM samples were tested with standard addition method. The light spot moved with the stage and stayed at the center of each site for 10s. The responses of standard and spiked samples are shown in Figure 1d and results are listed in Table 1, indicating that the multiplex ISLAPS can be applied in physiological ions detection. Table 1 The results of DMEM samples detection Spiked Sample 1 Spiked Sample2 Standard pCa True 2.301 1.699 2.744 Measurement 2.373 1.587 2.772 Relative Error 3.12% -6.59% 1.02% pNa True 0.699 0.398 0.809 Measurement 0.714 0.383 0.800 Relative Error 2.14% -3.80% -1.04% pK True 2 1.301 2.273 Measurement 2.070 1.217 2.345 Relative Error 3.50% -6.44% 3.168% pH True 7.861 7.696 7.913 Measurement 7.797 7.472 7.613 Relative Error -0.81% -2.91% -3.79% In summary, we have firstly combined silicone-rubber ISM with LAPS and proposed a multiplex ISLAPS system for Na + , K + , Ca 2+ and H + with good performance. The ISM-coated chips are packaged in one detection channel, and the response of multiple ions can be recorded sequentially. It seems to be a promising physiology detection platform. References [1] Hafeman D G, Parce J W, McConnell H M. Light-addressable potentiometric sensor for biochemical systems[J] . Science, 1988, 240(4856): 1182-1185. [2] Liang T, Gu C, Gan Y, et al. Microfluidic chip system integrated with light addressable potentiometric sensor (LAPS) for real-time extracellular acidification detection[J] . Sensors and Actuators B: Chemical, 2019, 301: 127004. [3] Ismail A B M, Harada T, Yoshinobu T, et al. Investigation of pulsed laser-deposited Al2O3 as a high pH-sensitive layer for LAPS-based biosensing applications[J] . Sensors and Actuators B: Chemical, 2000, 71(3): 169-172. Figure 1

Abstract: Introduction Urolithiasis is one of the most common diseases in the urinary system with very high incidence worldwide. In recent studies, citric acid (CA) has been considered as a biomarker of urolithiasis due to its vital suppression role in urinary stone formation. The detection of CA in urine is of great significance for early screening and prognostic monitoring of urolithiasis. However, most analytical methods for detecting CA are complicated and require expensive equipment. In the present study[1, 2], MIL-101(Fe) and Fe 3 O 4 were used for electrochemical detection of CA, based on the reaction of Fe(Ⅲ) and CA. Following these ideas, the electrodes modified with Fe 3 O 4 @MIL-101(Fe) nanoparticles were fabricated and investigated for whether they were able to detect CA and improve the sensitivity. Method Synthesis of Fe 3 O 4 @MIL-101(Fe) nanoparticles(Figure1. A): The Fe 3 O 4 @MIL-101(Fe) core-shell nanoparticles were synthesized according to previous literatures[3, 4]. Specifically, 3.6 g NaAC and 0.87 g FeCl 3 were dissolved in 75 mL ethylene glycol under ultrasound for 30 minutes. The mixture was transferred to a Teflon-lined autoclave chamber and heated at 200 °C for 16 h, then naturally cooled to room temperature. The obtained magnetic nanoparticles were collected with a magnet and washed with ethanol for three times. 350 mg above nanoparticles was dissolved to 75mL ethanol with 0.58mM Mercapto acetic acid (MAA), and gently stirred for 24h under nitrogen protection. The steps of self-assembly cycle were as follows. Firstly, 100 mg Fe 3 O 4 -MAA was dispersed in 10mM FeCl 3 , shaked for 1 minute and left for 15 minutes, then washed with ethanol for three times. Next, the nanoparticles was re-dispersed in 10 mM benzene-1,3,5-tricarboxylic acid, kept for 30 min at 70 °C with stripping, and then washed with ethanol for three times. After repeating this cycle 31 times, the synthesized core-shell nanoparticles were dried at 75 °C. Fabrication of the Fe 3 O 4 @MIL-101(Fe) /GCE: The GCE working electrode was polished with 1.0, 0.3, and 0.05 μm Al 2 O 3 substance and sonicated with an ethyl alcohol and water solution for 5 min respectively then dried under a N 2 stream. For the modification, the synthesized Fe 3 O 4 @MIL-101(Fe) core-shell nanoparticles was dissolved in ultrapure water and the final concentration was 1.0 mg/mL. After sonicating for 1 hour, 7 µL of the suspension was dripped onto the surface of the GCE and dried at room temperature. Detection of CA: Electrochemical analyses were carried out using differential pulse voltammetry (DPV) measurements in 0.01M KCl solution with different concentration of CA at pH 7.0. All experiments were performed at room temperature with a three-electrode system: Ag/AgCl (3 M KCl) electrode and platinum wire were used as a reference electrode and counter electrode, respectively, and applying in a potential range of 0 to 0.60 V, with an amplitude of 0.025 V and pulse period of 0.5 s. Results and Conclusions Characterization of Fe 3 O 4 @MIL-101(Fe): TEM images of as-prepared nanoparticles is shown in Figure 1. B. The Fe 3 O 4 @MIL-101(Fe) had spherical morphology and the core–shell structures which indicated the formation of MIL-101(Fe) layer on the Fe 3 O 4 nanoparticles.. The dark core was the Fe 3 O 4 with a diameter of about 348 nm, and the relatively bright outer layer was the MIL-101(Fe) with the thickness of 246 nm. The detection of CA: The DPVs of Fe 3 O 4 @MIL-101(Fe) /GCE in the 0.1M KCl with or without 100 mg/L CA are shown in Figure 1. C. In the presence of CA, a current peak was obtained in 0.324V. This could be explained due to the reaction of Fe 3+ ions in Fe 3 O 4 @MIL-101(Fe) with CA. The performance of the Fe 3 O 4 @MIL-101(Fe) /GCE in different concentrations of CA solutions were monitored by DPV, and the corresponding ΔCurrent( ) are shown in Figure 1.D. The resulting calibration is presented in the Figure 1. E, which was linear over the concentration range of 40 to 100 mg/L and 100-500 mg/L CA, with the linear regression equations of Y = 0.0004*X +0.004 (R 2 = 0.9956) and Y = 0.00016*X + 0.03773 (R 2 = 0.9766), respectively. Based on three times the background noise, the modified electrode provided a limit of detection of 31 mg/L. References Valizadeh, H., J. Tashkhourian, and A.J.M.A. Abbaspour, A carbon paste electrode modified with a metal-organic framework of type MIL-101 (Fe) for voltammetric determination of citric acid. 2019. 186 (7): p. 455. Guivar, J.A.R., et al., Preparation and characterization of cetyltrimethylammonium bromide (CTAB)-stabilized Fe3O4 nanoparticles for electrochemistry detection of citric acid. 2015. 755 : p. 158-166. Chen, Y., et al., Facile preparation of core–shell magnetic metal–organic framework nanoparticles for the selective capture of phosphopeptides. 2015. 7 (30): p. 16338-16347. Salman, F., A. Zengin, and H.Ç.J.I. Kazici, Synthesis and characterization of Fe 3 O 4-supported metal–organic framework MIL-101 (Fe) for a highly selective and sensitive hydrogen peroxide electrochemical sensor. 2020. 26 (10): p. 5221-5232. Figure 1