{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,23]],"date-time":"2026-01-23T13:34:17Z","timestamp":1769175257796,"version":"3.49.0"},"reference-count":77,"publisher":"MDPI AG","issue":"7","license":[{"start":{"date-parts":[[2014,7,22]],"date-time":"2014-07-22T00:00:00Z","timestamp":1405987200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/3.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>A new silica-gel nanospheres (SiO2NPs) composition was formulated, followed by biochemical surface functionalization to examine its potential in urea biosensor development. The SiO2NPs were basically synthesized based on sol\u2013gel chemistry using a modified Stober method. The SiO2NPs surfaces were modified with amine (-NH2) functional groups for urease immobilization in the presence of glutaric acid (GA)  cross-linker. The chromoionophore pH-sensitive dye ETH 5294 was physically adsorbed on the functionalized SiO2NPs as pH transducer. The immobilized urease determined urea concentration reflectometrically based on the colour change of the immobilized chromoionophore as a result of the enzymatic hydrolysis of urea. The pH changes on the biosensor due to the catalytic enzyme reaction of immobilized urease were found to correlate with the urea concentrations over a linear response range of 50\u2013500 mM  (R2 = 0.96) with a detection limit of 10 mM urea. The biosensor response time was 9 min with reproducibility of less than 10% relative standard deviation (RSD). This optical urea biosensor did not show interferences by Na+, K+, Mg2+ and NH4+ ions. The biosensor performance has been validated using urine samples in comparison with a non-enzymatic method based on the use of p-dimethylaminobenzaldehyde (DMAB) reagent and demonstrated a good correlation between the two different methods (R2 = 0.996 and regression slope of 1.0307). The SiO2NPs-based reflectometric urea biosensor showed improved dynamic linear response range when compared to other nanoparticle-based optical urea biosensors.<\/jats:p>","DOI":"10.3390\/s140713186","type":"journal-article","created":{"date-parts":[[2014,7,22]],"date-time":"2014-07-22T11:19:25Z","timestamp":1406027965000},"page":"13186-13209","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":46,"title":["A Large Response Range Reflectometric Urea Biosensor Made from Silica-Gel Nanoparticles"],"prefix":"10.3390","volume":"14","author":[{"given":"Muawia","family":"Alqasaimeh","sequence":"first","affiliation":[{"name":"School of Chemical Sciences and Food Technology, Faculty of Science and Technology,  Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor D.E., Malaysia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Lee","family":"Heng","sequence":"additional","affiliation":[{"name":"School of Chemical Sciences and Food Technology, Faculty of Science and Technology,  Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor D.E., Malaysia"},{"name":"Southeast Asia Disaster Prevention Research Initiative (SEADPRI-UKM), LESTARI,  Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor D.E., Malaysia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Musa","family":"Ahmad","sequence":"additional","affiliation":[{"name":"Industrial Chemical Technology Programme, Faculty of Science and Technology,  Universiti Sains Islam Malaysia, Bandar Baru Nilai, 71800 Nilai, Negeri Sembilan D.K.,  Malaysia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"A.S.","family":"Raj","sequence":"additional","affiliation":[{"name":"Electron Microscopy Unit, Institute for Medical Research, Jalan Pahang, 50588 Kuala Lumpur, Malaysia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Tan","family":"Ling","sequence":"additional","affiliation":[{"name":"Southeast Asia Disaster Prevention Research Initiative (SEADPRI-UKM), LESTARI,  Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor D.E., Malaysia"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2014,7,22]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"195","DOI":"10.1016\/S0003-2670(99)00699-6","article-title":"Sol-gel-derived thick-film conductometric biosensor for urea determination in serum","volume":"404","author":"Lee","year":"2000","journal-title":"Anal. Chim. Acta"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"585","DOI":"10.1016\/j.snb.2004.12.013","article-title":"Electrospun biocomposite nanofibers for urea biosensing","volume":"108","author":"Sawicka","year":"2005","journal-title":"Sens. Actuators B. Chem."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"3683","DOI":"10.1016\/j.biomaterials.2004.09.024","article-title":"An amperometric urea biosensor based on covalent immobilization of urease onto an electrochemically prepared copolymer poly(N-3-aminopropyl pyrrole-co-pyrrole) film","volume":"26","author":"Rajesh","year":"2005","journal-title":"Biomaterials"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"231","DOI":"10.1016\/j.surfcoat.2004.10.022","article-title":"A novel thin film urea biosensor based on copolymer poly(N-3-aminopropylpyrrole-co-pyrrole) film","volume":"198","author":"Rajesh","year":"2005","journal-title":"Surface Coat. Technol."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"51","DOI":"10.1016\/j.reactfunctpolym.2004.08.009","article-title":"Development of a potentiometric urea biosensor based on copolymer poly(N-3-aminopropyl pyrrole-co-pyrrole) film","volume":"62","author":"Rajesh","year":"2005","journal-title":"React. Funct. Polym."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"1157","DOI":"10.1016\/j.bios.2003.11.002","article-title":"Development of urease and glutamic dehydrogenase amperometric assay for heavy metals screening in polluted samples","volume":"19","author":"Rodriguez","year":"2004","journal-title":"Biosens. Bioelectron."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"148","DOI":"10.1016\/j.snb.2006.08.006","article-title":"Ion-sensitive field-effect transistor based pH sensors using nano self-assembled polyelectrolyte\/nanoparticle multilayer films","volume":"123","author":"Liu","year":"2007","journal-title":"Sens. Actuators B Chem."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"408","DOI":"10.1016\/j.snb.2006.04.047","article-title":"Novel routes for inter-matrix synthesis and characterization of polymer stabilized metal nanoparticles for molecular recognition devices","volume":"118","author":"Muraviev","year":"2006","journal-title":"Sens. Actuators B Chem."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"68","DOI":"10.1016\/j.aca.2010.07.030","article-title":"Nanoengineered optical urea biosensor for estimating hemodialysis parametersin spent dialysate","volume":"676","author":"Swati","year":"2010","journal-title":"Anal. Chim. Acta"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"53","DOI":"10.1016\/j.aca.2008.07.042","article-title":"Use of CdSe\/ZnS luminescent quantum dots incorporated within sol-gel matrix for urea detection","volume":"626","author":"Duong","year":"2008","journal-title":"Anal. Chim. Acta"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"2027","DOI":"10.1007\/s10008-010-1073-1","article-title":"Dual potentiometric and UV\/Vis spectrophotometric disposable sensors with dispersion cast polyaniline","volume":"14","author":"Malinowski","year":"2010","journal-title":"J. Solid State Electrochem."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"211","DOI":"10.1016\/j.jcis.2007.08.046","article-title":"Rapid evaporation-induced synthesis of monodisperse budded silica spheres","volume":"316","author":"Chen","year":"2007","journal-title":"J. Colloid Interface Sci."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"510","DOI":"10.1016\/j.jnoncrysol.2006.10.051","article-title":"Correlation between the luminescence properties and the surface structures of submicron silica particles","volume":"353","author":"Inai","year":"2007","journal-title":"J. Non-Crystalline Solids"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"93","DOI":"10.1016\/S0928-4931(02)00238-2","article-title":"Modified silica particles for gene delivery","volume":"23","author":"Csogor","year":"2003","journal-title":"Mater. Sci. Eng. C"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"203","DOI":"10.1016\/j.matchemphys.2005.09.003","article-title":"Further investigations on the modified Stober method for spherical MCM-41","volume":"97","author":"Liu","year":"2006","journal-title":"Mater. Chem. Phys."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"888","DOI":"10.1016\/j.talanta.2005.11.034","article-title":"An optical biosensor for dichlovos using stacked sol-gel films containing acetylcholinesterase and a lipophilic chromoionophore","volume":"69","author":"Wong","year":"2006","journal-title":"Talanta"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"1027","DOI":"10.1016\/S0039-9140(01)00590-2","article-title":"Nicotine derivative optode membrane with nonactin as ionophore","volume":"56","author":"Choi","year":"2002","journal-title":"Talanta"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"351","DOI":"10.1016\/j.solidstatesciences.2007.03.004","article-title":"Structural characterization of nanosized silica spheres","volume":"9","author":"Balas","year":"2007","journal-title":"Solid State Sci."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"62","DOI":"10.1016\/j.molcatb.2006.01.028","article-title":"Immobilization of Mucorjavanicus lipase on effectively functionalized silica nanoparticles","volume":"39","author":"Kim","year":"2006","journal-title":"J. Mol. Catal. B Enzym."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"4385","DOI":"10.1016\/j.polymer.2007.05.060","article-title":"Silica\/poly (N, N0-methylenebisacrylamide) composite materials by encapsulation based on a hydrogen-bonding interaction","volume":"48","author":"Liu","year":"2007","journal-title":"Polymer"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"2879","DOI":"10.1016\/j.matlet.2007.01.072","article-title":"Luminescence properties of Eu3+ or Dy3+\/Au co-doped SiO2 nanoparticles","volume":"61","author":"Petit","year":"2007","journal-title":"Mater. Lett."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"314","DOI":"10.1016\/j.jcis.2004.01.056","article-title":"Preparation of silica\u2013polystyrene core\u2013shell particles up to micron sizes","volume":"272","author":"Gu","year":"2004","journal-title":"J. Colloid Interface Sci."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"50","DOI":"10.1016\/j.molcatb.2009.12.003","article-title":"Super paramagnetic aminopropyl-functionalized silica core-shell microspheres as magnetically separable carriers for immobilization of penicillin G acylase","volume":"63","author":"Shi","year":"2010","journal-title":"J. Mol. Catal. B Enzym."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"4042","DOI":"10.1016\/j.biomaterials.2006.03.014","article-title":"Multilayered construction of glucose oxidase and silica nanoparticles on Au electrodes based on layer-by-layer covalent attachment","volume":"27","author":"Sun","year":"2006","journal-title":"Biomaterials"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"184","DOI":"10.1016\/j.cattod.2009.02.014","article-title":"Aminopropyl-functionalized silicas synthesized by W\/O microemulsion for immobilization of penicillin G acylase","volume":"148","author":"Shi","year":"2009","journal-title":"Catal. Today"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"79","DOI":"10.1016\/S1381-5148(02)00028-7","article-title":"Covalent immobilization of GL-7-ACA acylase on silica gel through silanization","volume":"51","author":"Park","year":"2002","journal-title":"React. Funct. Polym."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"385","DOI":"10.1016\/S0927-7757(97)00170-2","article-title":"Chemical immobilisation of humic acid on silica","volume":"141","author":"Koopal","year":"1998","journal-title":"Colloids Surfaces A Physicochem. Eng. Asp."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"127","DOI":"10.1016\/j.colsurfa.2004.03.004","article-title":"Synthesis of amino-silane modified superparamagnetic silica supports and their use for protein immobilization","volume":"238","author":"Liu","year":"2004","journal-title":"Colloids Surfaces A Physicochem. Eng. Asp."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"179","DOI":"10.1016\/j.jmmm.2009.07.075","article-title":"Surface functionalization of silica-coated magnetic nanoparticles for covalent attachment of cholesterol oxidase","volume":"322","author":"Drofenik","year":"2010","journal-title":"J. Magn. Magn. Mater."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"181","DOI":"10.1016\/S0003-2670(03)00123-5","article-title":"Tris(2,2\u2032-bipyridyl)cobalt(III)-doped silica nanoparticle DNA probe for the electrochemical detection of DNA hybridization","volume":"481","author":"Zhu","year":"2003","journal-title":"Anal. Chim. Acta"},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"302","DOI":"10.1006\/abio.1996.0171","article-title":"Linearization of the bradford protein assay increases its sensitivity: Theoretical and experimental studies","volume":"36","author":"Zor","year":"1996","journal-title":"Anal. Biochem."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"248","DOI":"10.1016\/0003-2697(76)90527-3","article-title":"A dye binding assay for protein","volume":"72","author":"Bradford","year":"1976","journal-title":"Anal. Biochem."},{"key":"ref_33","unstructured":"Williams, S. (1984). Official Methods of Analysis of the Association of Official Analytical Chemists, The Association of Official Analytical Chemists Inc."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"286","DOI":"10.1016\/j.jnoncrysol.2006.12.006","article-title":"Silica xerogels of tailored porosity as support matrix for optical chemical sensors. Simultaneous effect of pH, ethanol:TEOS and water:TEOS molar ratios, and synthesis temperature on gelation time, and textural and structural properties","volume":"353","author":"Estella","year":"2007","journal-title":"J. Non-Crystalline Solids"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"169","DOI":"10.1016\/j.snb.2004.04.049","article-title":"Development of a sol-gel optical sensor for analysis of zinc in pharmaceuticals","volume":"103","author":"Montenegro","year":"2004","journal-title":"Sens. Actuators B"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"55","DOI":"10.1016\/S0731-7085(98)00129-0","article-title":"Chemical and biochemical sensors based on advances in materials chemistry","volume":"19","year":"1999","journal-title":"J. Pharm. Biomed. Anal."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"13","DOI":"10.1016\/j.talanta.2006.09.029","article-title":"Optical sensors and biosensors based on sol-gel films","volume":"72","author":"Araujo","year":"2007","journal-title":"Talanta"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"450","DOI":"10.1016\/j.colsurfa.2007.11.020","article-title":"Facile synthesis of Fe3O4\/SiO2 composite nanoparticles from primary silica particles","volume":"317","author":"Lu","year":"2008","journal-title":"Colloids Surfaces A Physicochem. Eng. Asp."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"208","DOI":"10.1016\/j.jcis.2011.10.070","article-title":"Stober silica particles as basis for redox modifications: Particle shape, size, polydispersity, and porosity","volume":"368","author":"Ruff","year":"2012","journal-title":"J. Colloid Interface Sci."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"510","DOI":"10.1016\/j.matchemphys.2012.05.016","article-title":"Hydrothermal assisted synthesis of iron oxide-based magnetic silica spheres and their performance in magnetophoretic water purification","volume":"135","author":"Benelmekki","year":"2012","journal-title":"Mater. Chem. Phys."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"16","DOI":"10.1016\/j.jcis.2011.05.038","article-title":"Synthesis and characterization of pore size-tunable magnetic mesoporous silica nanoparticles","volume":"361","author":"Zhang","year":"2011","journal-title":"J. Colloid Interface Sci."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"2261","DOI":"10.1016\/j.matlet.2011.04.079","article-title":"Preparation of helical mesoporous ethylene-silica nanofibers using CTAB and (S)-2-methyl-1-butanol","volume":"65","author":"Wang","year":"2011","journal-title":"Mater. Lett."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"135","DOI":"10.1016\/j.cocis.2011.01.004","article-title":"Nanoseparations: Strategies for size and\/or shape-selective purification of nanoparticles","volume":"16","author":"Kowalczyk","year":"2011","journal-title":"Curr. Opin. Colloid Interface Sci."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"43","DOI":"10.1016\/j.jlumin.2006.05.001","article-title":"Multi-peak behavior of photoluminescence of silica particles heat-treated in hydrogen at elevated temperature","volume":"126","author":"Xu","year":"2007","journal-title":"J. Lumin."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"1266","DOI":"10.1016\/j.optmat.2004.11.022","article-title":"Organic-inorganic hybrid coating (poly(methyl methacrylate)\/monodisperse silica)","volume":"27","author":"Rubio","year":"2005","journal-title":"Opt. Mater."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"38","DOI":"10.1016\/j.cej.2007.09.012","article-title":"Comparative study of solid silica nanoparticles and hollow silica nanoparticles for the immobilization of lysozyme","volume":"137","author":"Xiao","year":"2008","journal-title":"Chem. Eng. J."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"81","DOI":"10.1016\/j.jcis.2008.08.054","article-title":"Structural and thermal characterizations of silica nanoparticles grafted with pendant maleimide and epoxide groups","volume":"328","author":"Vejayakumaran","year":"2008","journal-title":"J. Colloid Interface Sci."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"241","DOI":"10.1016\/j.matchemphys.2007.11.027","article-title":"Ammonium carboxylates assisted combustion process for the synthesis of nanocrystalline LiCoO2 powders","volume":"109","author":"Vivekanandhan","year":"2008","journal-title":"Mater. Chem. Phys."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"90","DOI":"10.1016\/j.msec.2010.08.001","article-title":"Potentiometric urea biosensor based on multi-walled carbon nanotubes(MWCNTs)\/silica composite material","volume":"31","author":"Ahuja","year":"2011","journal-title":"Mater. Sci. Eng. C"},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"5604","DOI":"10.1016\/S1452-3981(23)19647-7","article-title":"Complex Impedance Spectroscopy Study of Silica Nanoparticles Via Sol-Gel Method","volume":"7","author":"Singho","year":"2012","journal-title":"Int. J. Electrochem. Sci."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"517","DOI":"10.1016\/j.jiec.2010.03.030","article-title":"Surface modification of silica nanoparticles with hydrophilic polymers","volume":"16","author":"Park","year":"2010","journal-title":"J. Ind. Eng. Chem."},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"252","DOI":"10.1016\/j.powtec.2012.01.009","article-title":"Synthesis of surface functionalized silica nanoparticles and their use as entomotoxic nanocides","volume":"221","author":"Debnath","year":"2012","journal-title":"Powder Technol."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"128","DOI":"10.1016\/j.matchemphys.2008.01.042","article-title":"Synthesis of CeO2-coated SiO2 nanoparticle and dispersion stability of its suspension","volume":"110","author":"Song","year":"2008","journal-title":"Mater. Chem. Phys."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"44","DOI":"10.5772\/50950","article-title":"Preparation of Silica Nanoparticles and Its Beneficial Role in Cementitious Materials","volume":"1","author":"Singh","year":"2011","journal-title":"Nanomater. Nanotechnol."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"1720","DOI":"10.1016\/j.matlet.2010.05.010","article-title":"Synthesis and enhanced electrochemical properties of pod-shaped gold\/silica nanoparticles","volume":"64","author":"Ye","year":"2010","journal-title":"Mater. Lett."},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"91","DOI":"10.1016\/j.aca.2004.01.028","article-title":"Optical determination of ionophore diffusion coefficients in plasticized poly(vinyl chloride) sensing films","volume":"511","author":"Long","year":"2004","journal-title":"Anal. Chim. Acta"},{"key":"ref_57","doi-asserted-by":"crossref","first-page":"2251","DOI":"10.3390\/s7102251","article-title":"A Urea Biosensor from Stacked Sol-Gel Films with Immobilized Nile Blue Chromoionophore and Urease Enzyme","volume":"7","author":"Alqasaimeh","year":"2007","journal-title":"Sensors"},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"105","DOI":"10.1016\/j.snb.2008.11.011","article-title":"An optical fibre reflectance sensor for p-aminophenol determination based ontetrahydroxycalix[4]arene as sensing reagent","volume":"136","author":"Filik","year":"2009","journal-title":"Sens. Actuators B Chem."},{"key":"ref_59","doi-asserted-by":"crossref","first-page":"103","DOI":"10.1016\/j.talanta.2008.05.045","article-title":"Development of an optical fibre reflectance sensor for p-aminophenol detection based on immobilised bis-8-hydroxyquinoline","volume":"77","author":"Filik","year":"2008","journal-title":"Talanta"},{"key":"ref_60","doi-asserted-by":"crossref","first-page":"194","DOI":"10.1016\/j.snb.2004.12.043","article-title":"Potentiometric enzyme electrode for urea determination using immobilized urease in poly(vinylferrocenium) film","volume":"109","author":"Kuralay","year":"2005","journal-title":"Sens. Actuators B Chem."},{"key":"ref_61","doi-asserted-by":"crossref","first-page":"1259","DOI":"10.1016\/0956-5663(96)88091-2","article-title":"A preliminary study of a biosensor based on flow injection of the recognition element","volume":"11","author":"Kriz","year":"1996","journal-title":"Biosens. Bioelectron."},{"key":"ref_62","doi-asserted-by":"crossref","first-page":"111","DOI":"10.1016\/S0956-5663(02)00164-1","article-title":"Optical biosensor for urea with improved response time","volume":"18","author":"Kovacs","year":"2003","journal-title":"Biosens. Bioelectron."},{"key":"ref_63","doi-asserted-by":"crossref","first-page":"435","DOI":"10.1016\/0956-5663(96)82739-4","article-title":"urea sensor based on combining the surface acoustic wave device with urease extracted from green soya bean and its application-determination of urea in human urine","volume":"11","author":"Liu","year":"1996","journal-title":"Biosens. Bioelectron."},{"key":"ref_64","doi-asserted-by":"crossref","first-page":"3283","DOI":"10.1016\/j.bios.2007.03.006","article-title":"Piezoelectric urea biosensor based on immobilization of urease onto nanoporous alumina membranes","volume":"22","author":"Yang","year":"2007","journal-title":"Biosens. Bioelectron."},{"key":"ref_65","doi-asserted-by":"crossref","first-page":"813","DOI":"10.1016\/j.bios.2003.08.007","article-title":"Comparative study of controlled pore glass, silica gel and Poraver for the immobilization of urease to determine urea in a flow injection conductimetric biosensor system","volume":"19","author":"Limbut","year":"2004","journal-title":"Biosens. Bioelectron."},{"key":"ref_66","doi-asserted-by":"crossref","first-page":"49","DOI":"10.1016\/j.snb.2010.09.051","article-title":"Gold nanoparticles-coated eggshell membrane with immobilized glucose oxidase for fabrication of glucose biosensor","volume":"152","author":"Zheng","year":"2011","journal-title":"Sens. Actuators B Chem."},{"key":"ref_67","doi-asserted-by":"crossref","first-page":"39","DOI":"10.1016\/S1369-703X(97)00007-7","article-title":"A kinetic approach to oxidase based enzyme electrodes: The effect of enzyme layer formation on the response time","volume":"1","author":"Mutlu","year":"1998","journal-title":"Biochem. Eng. J."},{"key":"ref_68","doi-asserted-by":"crossref","first-page":"1245","DOI":"10.1016\/j.talanta.2010.02.016","article-title":"The influence of external factors on the operational stability of the biosensor Response","volume":"81","author":"Stikoniene","year":"2010","journal-title":"Talanta"},{"key":"ref_69","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1016\/j.semcdb.2009.01.009","article-title":"A review of the use of genetically engineered enzymes in electrochemical biosensors","volume":"20","author":"Campas","year":"2009","journal-title":"Semin. Cell Dev. Biol."},{"key":"ref_70","doi-asserted-by":"crossref","first-page":"503","DOI":"10.1016\/S0956-5663(02)00021-0","article-title":"Enhancement of operational stability of an enzyme biosensor for glucose and sucrose using protein based stabilizing agents","volume":"17","author":"Gouda","year":"2002","journal-title":"Biosens. Bioelectron."},{"key":"ref_71","doi-asserted-by":"crossref","first-page":"489","DOI":"10.1016\/j.biotechadv.2011.09.003","article-title":"Research review paper Immobilization strategies to develop enzymatic biosensors","volume":"30","author":"Sassolas","year":"2012","journal-title":"Biotechnol. Adv."},{"key":"ref_72","doi-asserted-by":"crossref","first-page":"9","DOI":"10.1016\/j.molcatb.2009.01.003","article-title":"Functional, catalytic and kinetic properties: A review","volume":"59","author":"Krajewska","year":"2009","journal-title":"J. Mol. Catal. B Enzym."},{"key":"ref_73","doi-asserted-by":"crossref","first-page":"181","DOI":"10.1016\/S0009-8981(99)00240-5","article-title":"Rapid analysis of cation constituents of urine by capillary electrophoresis","volume":"293","author":"Olsauskaite","year":"2000","journal-title":"Clin. Chim. Acta"},{"key":"ref_74","doi-asserted-by":"crossref","first-page":"8323","DOI":"10.3390\/s110908323","article-title":"A Biosensor for Urea from Succinimide-Modified Acrylic Microspheres Based on Reflectance Transduction","volume":"1","author":"Ulianas","year":"2011","journal-title":"Sensors"},{"key":"ref_75","doi-asserted-by":"crossref","first-page":"165","DOI":"10.1016\/j.nantod.2009.02.001","article-title":"Mesoporous materials for encapsulating enzymes","volume":"4","author":"Lee","year":"2009","journal-title":"Nano Today"},{"key":"ref_76","doi-asserted-by":"crossref","first-page":"1289","DOI":"10.1002\/adsc.200700082","article-title":"Enzyme Immobilization: The Quest for Optimum Performance","volume":"349","author":"Sheldon","year":"2007","journal-title":"Adv. Synth. Catal."},{"key":"ref_77","doi-asserted-by":"crossref","first-page":"453","DOI":"10.1039\/B711564B","article-title":"Understanding enzyme immobilization","volume":"38","author":"Hanefeld","year":"2009","journal-title":"R. Soc. 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