Skip to main content
SpringerLink
Log in

Protein Engineering for Biosensors

  • Chapter
Body Sensor Networks

This is a preview of subscription content, log in via an institution to check access.

Access this chapter

Log in via an institution
Chapter
USD 29.95
Price excludes VAT (USA)
  • Available as PDF
  • Read on any device
  • Instant download
  • Own it forever
eBook
USD 89.00
Price excludes VAT (USA)
  • Available as PDF
  • Read on any device
  • Instant download
  • Own it forever

Tax calculation will be finalised at checkout

Purchases are for personal use only

Institutional subscriptions

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Cooper J, Cass T. Biosensors: a practical approach, 2nd ed. Oxford: Oxford University Press, 2004.

    Google Scholar 

  2. Wilson GS, Gifford R. Biosensors for real-time in vivo measurements. Biosensors and Bioelectronics 2005; 20(12):2388–2403.

    Article  Google Scholar 

  3. Schoning MJ. “Playing around” with field-effect sensors on the basis of EIS structures, LAPS and ISFETs. Sensors 2005; 5(3):126–138.

    Article  Google Scholar 

  4. Shepherd L, Toumazou C. Weak Inversion ISFETs for ultra-low power biochemical sensing and real-time analysis. Sensors and Actuators B: Chemical 2005; 107(1):468–473.

    Article  Google Scholar 

  5. Forrow NJ, Sanghera GS, Walters SJ. The influence of structure in the reaction of electrochemically generated ferrocenium derivatives with reduced glucose-oxidase. Journal of the Chemical Society-Dalton Transactions 2002; (16):3187–3194.

    Article  Google Scholar 

  6. Zhang WJ, Li GX. Third-generation biosensors based on the direct electron transfer of proteins. Analytical Sciences 2004; 20(4):603–609.

    Article  Google Scholar 

  7. Choi MMF. Progress in enzyme-based biosensors using optical transducers. Microchimica Acta 2004; 148(3–4):107–132.

    Article  Google Scholar 

  8. Gauglitz G. Direct optical sensors: principles and selected applications. Analytical and Bioanalytical Chemistry 2005; 381(1):141–155.

    Article  Google Scholar 

  9. Monk DJ, Walt DR. Optical fiber-based biosensors. Analytical and Bioanalytical Chemistry 2004; 379(7–8):931–945.

    Google Scholar 

  10. Taitt CR, Anderson GP, Ligler FS. Evanescent wave fluorescence biosensors. Biosensors and Bioelectronics 2005; 20(12):2470–2487.

    Article  Google Scholar 

  11. Zourob M, Mohr S, Brown BJT, Fielden PR, McDonnell MB, Goddard NJ. An integrated metal clad leaky waveguide sensor for detection of bacteria. Analytical Chemistry 2005; 77(1):232–242.

    Article  Google Scholar 

  12. Vaseashta A, Dimova-Malinovska D. Nanostructured and nanoscale devices, sensors and detectors. Science and Technology of Advanced Materials 2005; 6(3–4):312–318.

    Google Scholar 

  13. Jain KK. Nanotechnology in clinical laboratory diagnostics. Clinica Chimica Acta 2005; 358(1–2):37–54.

    Article  Google Scholar 

  14. Wang J. Nanomaterial-based amplified transduction of biomolecular interactions. Small 2005; 1(11):1036–1043.

    Article  Google Scholar 

  15. Nath N, Chilkoti A. Label free colorimetric biosensing using nanoparticles. Journal of Fluorescence 2004; 14(4):377–389.

    Article  Google Scholar 

  16. Pattnaik P. Surface plasmon resonance — applications in understanding receptor-ligand interaction. Applied Biochemistry and Biotechnology 2005; 126(2):79–92.

    Article  Google Scholar 

  17. Rickert J, Brecht A, Gopel W. Quartz crystal microbalances for quantitative biosensing and characterizing protein multilayers. Biosensors and Bioelectronics 1997; 12(7):567–575.

    Article  Google Scholar 

  18. Su XL, Li YB. A self-assembled monolayer-based piezoelectric immunosensor for rapid detection of Escherichia coli O157: H7. Biosensors and Bioelectronics 2004; 19(6):563–574.

    Article  Google Scholar 

  19. Berkenpas E, Bitla S, Millard P, da Cunha MP. Pure shear horizontal SAW biosensor on langasite. IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 2004; 51(11):1404–1411.

    Article  Google Scholar 

  20. Schlensog MD, Gronewold TMA, Tewes M, Famulok M, Quandt E. A Lovewave biosensor using nucleic acids as ligands. Sensors and Actuators BChemical 2004; 101(3):308–315.

    Article  Google Scholar 

  21. Godber B, Thompson KSJ, Rehak M, Uludag Y, Kelling S, Sleptsov A, et al. Direct quantification of analyte concentration by resonant acoustic profiling. Clinical Chemistry 2005; 51(10):1962–1972.

    Article  Google Scholar 

  22. Baier V, Fodisch R, Ihring A, Kessler E, Lerchner J, Wolf G, et al. Highly sensitive thermopile heat power sensor for micro-fluid calorimetry of biochemical processes. Sensors and Actuators A: Physical 2005; 123–124:354–359.

    Google Scholar 

  23. Johannessen EA, Weaver JMR, Bourova L, Svoboda P, Cobbold PH, Cooper JM. Micromachined nanocalorimetric sensor for ultra-low-volume cell-based assays. Analytical Chemistry 2002; 74(9):2190–2197.

    Article  Google Scholar 

  24. Callan JF, de Silva AP, Magri DC. Luminescent sensors and switches in the early 21st century. Tetrahedron 2005; 61(36):8551–8588.

    Article  Google Scholar 

  25. Hellinga HW, Marvin JS. Protein engineering and the development of generic biosensors. Trends in Biotechnology 1998; 16(4):183–189.

    Article  Google Scholar 

  26. Goodsell DS. Bionanotechnology. Hoboken: Wiley-Liss, 2004.

    Google Scholar 

  27. Gilardi G. Protein Engineering for Biosensors. In: Cooper J, Cass T (eds) Biosensors: A Practical Approach. Oxford: Oxford University Press, 2004.

    Google Scholar 

  28. Lutz S, Patrick WM. Novel methods for directed evolution of enzymes: quality, not quantity. Current Opinion in Biotechnology 2004; 15(4):291–297.

    Article  Google Scholar 

  29. Gilardi G, Fantuzzi A. Manipulating redox systems: application to nanotechnology. Trends in Biotechnology 2001; 19:468–476.

    Article  Google Scholar 

  30. Gilardi G, Zhou LQ, Hibbert L, Cass AEG. Engineering the maltose-binding protein for reagentless fluorescence sensing. Analytical Chemistry 1994; 66(21):3840–3847.

    Article  Google Scholar 

  31. Brune M, Hunter JL, Corrie JET, Webb MR. Direct, real-time measurement of rapid inorganic phosphate release using a novel fluorescent probe and its applications to actinomyosin subfragment 1 ATPase. Biochemistry 1994; 33:8262–8271.

    Article  Google Scholar 

  32. Gilardi G, Mei G, Rosato N, Agro AF, Cass AEG. Spectroscopic properties of an engineered maltose binding protein. Protein Engineering 1997; 10(5):479–486.

    Article  Google Scholar 

  33. Brune M, Hunter JL, Howell SA, Martin SR, Hazlett TL, Corrie JET, et al. Mechanism of inorganic phosphate interaction with phosphate binding protein from Escherichia coli. Biochemistry 1998; 37(29):10370–10380.

    Article  Google Scholar 

  34. Hirshberg M, Henrick K, Haire LL, Vasisht N, Brune M, Corrie JET, et al. Crystal structure of phosphate binding protein labeled with a coumarin fluorophore, a probe for inorganic phosphate. Biochemistry 1998; 37(29):10381–10385.

    Article  Google Scholar 

  35. Dattelbaum JD, Looger LL, Benson DE, Sali KM, Thompson RB, Hellinga HW. Analysis of allosteric signal transduction mechanisms in an engineered fluorescent maltose biosensor. Protein Science 2005; 14(2):284–291.

    Article  Google Scholar 

  36. Allert M, Rizk SS, Looger LL, Hellinga HW. Computational design of receptors for an organophosphate surrogate of the nerve agent soman. Proceedings of the National Academy of Sciences of the United States of America 2004; 101(21):7907–7912.

    Article  Google Scholar 

  37. De Lorimier RM, Smith JJ, Dwyer MA, Looger LL, Sali KM, Paavola CD,et al. Construction of a fluorescent biosensor family. Protein Science 2002; 11(11):2655–2675.

    Article  Google Scholar 

  38. Marvin JS, Hellinga HW. Conversion of a maltose receptor into a zinc biosensor by computational design. Proceedings of the National Academy of Sciences of the United States of America 2001; 98(9):4955–4960.

    Article  Google Scholar 

  39. Marvin JS, Corcoran EE, Hattangadi NA, Zhang JV, Gere SA, Hellinga HW. The rational design of allosteric interactions in a monomeric protein and its applications to the construction of biosensors. Proceedings of the National Academy of Sciences of the United States of America 1997; 94(9):4366–4371.

    Article  Google Scholar 

  40. Badugu R, Lakowicz JR, Geddes CD. Ophthalmic glucose monitoring using disposable contact lenses — a review. Journal of Fluorescence 2004; 14(5):617–633.

    Article  Google Scholar 

  41. Renard M, Bedouelle H. Improving the sensitivity and dynamic range of reagentless fluorescent immunosensors by knowledge-based design. Biochemistry 2004; 43(49):15453–15462.

    Article  Google Scholar 

  42. Chan PH, Liu HB, Chen YW, Chan KC, Tsang CW, Leung YC, et al. Rational design of a novel fluorescent biosensor for beta-lactam antibiotics from a class A beta-lactamase. Journal of the American Chemical Society 2004; 126(13):4074–4075.

    Article  Google Scholar 

  43. Nagase T, Nakata E, Shinkai S, Hamachi I. Construction of artificial signal transducers on a lectin surface by post-photoaffinity-labeling modification for fluorescent saccharide biosensors. Chemistry 2003; 9(15):3660–3669.

    Article  Google Scholar 

  44. Benson DE, Conrad DW, de Lorimier RM, Trammell SA, Hellinga HW. Design of bioelectronic interfaces by exploiting hinge-bending motions in proteins. Science 2001; 293(5535):1641–1644.

    Article  Google Scholar 

  45. Kase Y, Muguruma H. Amperometric glucose biosensor based on mediated electron transfer between immobilized glucose oxidase and plasma-polymerized thin film of dimethylaminomethylferrocene on sputtered gold electrode. Analytical Sciences 2004; 20(8):1143–1146.

    Article  Google Scholar 

  46. Battaglini F, Bartlett PN, Wang JH. Covalent attachment of osmium complexes to glucose oxidase and the application of the resulting modified enzyme in an enzyme switch responsive to glucose. Analytical Chemistry 2000; 72(3):502–509.

    Article  Google Scholar 

  47. Sadeghi SJ, G. Gilardi, Cass AEG. Mediated electrochemistry of peroxidases-effects of variations in protein and mediator structures. Biosensors and Bioelectronics 1997; 12(12):1191–1198.

    Article  Google Scholar 

  48. Loechel C, Basran A, Basran J, Scrutton NS, Hall EAH. Using trimethylamine dehydrogenase in an enzyme linked amperometric electrode II: rational design engineering of a ‘wired’ mutant. Analyst 2003; 128(7):889–898.

    Article  Google Scholar 

  49. Chen LQ, Zhang XE, Xie WH, Zhou YF, Zhang ZP, Cass AEG. Genetic modification of glucose oxidase for improving performance of an amperometric glucose biosensor. Biosensors and Bioelectronics 2002; 17(10):851–857.

    Article  Google Scholar 

  50. Degani Y, Heller A. Direct electrical communication between chemically modified enzymes and metal-electrodes I: electron-transfer from glucose-oxidase to metal-electrodes via electron relays, bound covalently to the enzyme. Journal of Physical Chemistry 1987; 91(6):1285–1289.

    Article  Google Scholar 

  51. Easterby JS. The analysis of metabolite channelling in multienzyme complexes and multifunctional proteins. Biochemical Journal 1989; 264(2):605–607.

    Google Scholar 

  52. Zhou YF, Zhang XE, Liu H, Zhang ZP, Zhang CG, Cass AEG. Construction of a fusion enzyme system by gene splicing as a new molecular recognition element for a sequence biosensor. Bioconjugate Chemistry 2001; 12(6):924–931.

    Article  Google Scholar 

  53. Martineau P, Szmelcman S, Spurlino JC, Quiocho FA, Hofnung M. Genetic approach to the role of tryptophan residues in the activities and fluorescence of a bacterial periplasmic maltose-binding protein. Journal of Molecular Biology 1990; 214(1):337–352.

    Article  Google Scholar 

  54. Fierke CA, Thompson RB. Fluorescence-based biosensing of zinc using carbonic anhydrase. Biometals 2001; 14(3–4):205–222.

    Article  Google Scholar 

  55. Soldatkin AP, Montoriol J, Sant W, Martelet C, Jaffrezic-Renault N. A novel urea sensitive biosensor with extended dynamic range based on recombinant urease and ISFETs. Biosensors and Bioelectronics 2003; 19(2):131–135.

    Article  Google Scholar 

  56. Xu HF, Zhang XE, Zhang ZP, Zhang YM, Cass AEG. Directed evolution of e-coli alkaline phosphatase towards higher catalytic activity. Biocatalysis and Biotransformation 2003; 21(1):41–47.

    Article  Google Scholar 

  57. Zeng HH, Thompson RB, Maliwal BP, Fones GR, Moffett JW, Fierke CA. Real-time determination of picomolar free Cu(II) in seawater using a fluorescence based fiber optic biosensor. Analytical Chemistry 2003; 75(24):6807–6812.

    Article  Google Scholar 

  58. Sacchi S, Rosini E, Molla G, Pilone MS, Pollegioni L. Modulating D-amino acid oxidase substrate specificity: production of an enzyme for analytical determination of all D-amino acids by directed evolution. Protein Engineering Design and Selection 2004; 17(6):517–525.

    Article  Google Scholar 

  59. Boublik Y, Saint-Aguet P, Lougarre A, Arnaud M, Villatte F, Estrada-Mondaca S, et al. Acetylcholinesterase engineering for detection of insecticide residues. Protein Engineering 2002; 15(1):43–50.

    Article  Google Scholar 

  60. Schulze H, Muench SB, Villatte F, Schmid RD, Bachmann TT. Insecticide detection through protein engineering of Nippostrongylus brasiliensis acetylcholinesterase B. Analytical Chemistry 2005; 77(18):5823–5830.

    Article  Google Scholar 

  61. Flower DR, North ACT, Sansom CE. The lipocalin protein family: structure and sequence overview. Biochimica et Biophysica Acta-Protein Structure and Molecular Enzymology 2000; 1482(1–2):9–24.

    Article  Google Scholar 

  62. Schlehuber S, Skerra A. Lipocalins in drug discovery: from natural ligandbinding proteins to ‘anticalins’. Drug Discovery Today 2005; 10(1):23–33.

    Article  Google Scholar 

  63. Korndorfer IP, Beste G, Skerra A. Crystallographic analysis of an “anticalin” with tailored specificity for fluorescein reveals high structural plasticity of the lipocalin loop region. Proteins-Structure Function and Genetics 2003; 53(1):121–129.

    Article  Google Scholar 

  64. Mercader JV, Skerra A. Generation of anticalins with specificity for a non-symmetric phthalic acid ester. Analytical Biochemistry 2002; 308(2):269–277.

    Article  Google Scholar 

  65. Dwyer MA, Hellinga HW. Periplasmic binding proteins: a versatile super-family for protein engineering. Current Opinion in Structural Biology 2004; 14(4):495–504.

    Article  Google Scholar 

  66. Looger LL, Dwyer MA, Smith JJ, Hellinga HW. Computational design of receptor and sensor proteins with novel functions. Nature 2003; 423(6936):185–190.

    Article  Google Scholar 

  67. Stroscio MA, Dutta M. Integrated biological-semiconductor devices. Proceedings of the IEEE 2005; 93(10):1772–1783.

    Article  Google Scholar 

  68. Sheehan PE, Whitman LJ. Detection limits for nanoscale biosensors. Nano Letters 2005; 5(4):803–807.

    Article  Google Scholar 

  69. Halliwell CM. Nanoanalytical measurement of protein orientation on conductive sensor surfaces. Analyst 2004; 129(12):1166–1170.

    Article  Google Scholar 

  70. Davis J, Glidle A, Cass AEG, Zhang JK, Cooper JM. Spectroscopic evaluation of protein affinity binding at polymeric biosensor films. Journal of the American Chemical Society 1999; 121(17):4302–4303.

    Article  Google Scholar 

  71. Zhang JK, Cass AEG. A study of his-tagged alkaline phosphatase immobilization on a nanoporous nickel-titanium dioxide film. Analytical Biochemistry 2001; 292(2):307–310.

    Article  Google Scholar 

  72. Ferapontova E, Gorton L. Bioelectrocatalytical detection of H2O2 with different forms of horseradish peroxidase directly adsorbed at polycrystalline silver and gold. Electroanalysis 2003; 15(5–6):484–491.

    Article  Google Scholar 

  73. Shi JX, Zhang XE, Xie WH, Zhou YF, Zhang ZP, Deng JY, et al. Improvement of homogeneity of analytical biodevices by gene manipulation. Analytical Chemistry 2004; 76(3):632–638.

    Article  Google Scholar 

  74. Halliwell CM, Simon E, Toh CS, Bartlett PN, Cass AEG. A method for the determination of enzyme mass loading on an electrode surface through radioisotope labelling. Biosensors and Bioelectronics 2002; 17(11–12):965–972.

    Article  Google Scholar 

  75. Shao W-H, Zhang X-E, Liu H, Zhang ZP, Cass AEG. An ‘anchor-chain’ molecular system for orientation control of enzyme immobilization with high recovery of activity. Bioconjugate Chemistry 2000; 11:822–826.

    Article  Google Scholar 

  76. Nygren H, Stenberg M. Surface-induced aggregation of ferritin. Kinetics of adsorption to a hydrophobic surface. Biophysical Chemistry 1990; 38:67–75.

    Article  Google Scholar 

  77. Sugihara T, Seong GH, Kobatake E, Aizawa M. Genetically synthesized antibody-binding protein self-assembled on hydrophobic matrix. Bioconjugate Chemistry 2000; 11(6):789–794.

    Article  Google Scholar 

  78. Wada A, Mie M, Aizawa M, Lahoud P, Cass AEG, Kobatake E. Design and construction of glutamine binding proteins with a self-adhering capability to unmodified hydrophobic surfaces as reagentless fluorescence sensing devices. Journal of the American Chemical Society 2003; 125(52):16228–16234.

    Article  Google Scholar 

  79. Haberzettl CA. Nanomedicine: destination or journey? Nanotechnology 2002; 13:R9–R13.

    Article  Google Scholar 

  80. Malhotra BD, Chaubey A. Biosensors for clinical diagnostics industry. Sensors and Actuators B 2003; 91:117–127.

    Article  Google Scholar 

  81. Renard E. Implanted closed-loop glucose-sensing and insulin delivery: the future for insulin pump therapy. Current Opinion in Pharmacology 2002: 708–716.

    Google Scholar 

  82. Woedtke Tv, Jülich W-D, Hartmann V, Stieber M, Abel PU. Sterilisation of enzyme glucose sensors: problems and concepts. Biosensors and Bioelectronics 2002; 17:373–382.

    Article  Google Scholar 

  83. Abel PU, Woedtke TV. Biosensors for in vivo glucose measurements: can we cross the experimental stage? Biosensors and Bioelectronics 2002; 17:1059–1070.

    Article  Google Scholar 

  84. Frost MC, Meyerhoff ME. Implantable chemical sensors for real-time clinical monitoring: progress and challenges. Current Opinion in Chemical Biological 2002; 6:633–641.

    Article  Google Scholar 

  85. Wisniewski N, Moussy F, Reichert WM. Characterization of implantable biosensor membrane biofouling. Fresenius’ Journal of Analytical Chemistry 2000; 366:611–621

    Article  Google Scholar 

  86. Atanasov P, Yang S, Salehi C, Ghindilis AL, Wilkins E, Schade D. Implantation of refillable glucose monitoring-telemetry device. Biosensors and Bioelectronics 1997; 12:669–680.

    Article  Google Scholar 

  87. Henry C. Getting under the skin: implantable electrochemical glucose sensors are moving closer to commercialization. Analytical Chemistry 1998; 60:594A–598A.

    Google Scholar 

  88. Kerner W, Kiwit M, Linke B, Keck FS, Zier H, Pfeiffer EF. The function of a hydrogen peroxide-detecting electroenzymatic glucose electrode is markedly impaired in human subcutaneous tissue and plasma. Biosensors and Bioelectronics 1993; 8:473–482.

    Article  Google Scholar 

  89. Moussy F, Harrison DJ, O’Brien DW, Rajotte RV. Performance of subcutaneously implanted needle-type glucose sensor employing a novel trilayer coating. Analytical Chemistry 1993; 65:2072–2077.

    Article  Google Scholar 

  90. Ammon HP, Ege W, Oppermann M, Gopel W, Eisele S. Improvement in the long-term stability of an amperometric glucose sensor system by introducing a cellulose membrane of bacterial origin. Analytical Chemistry 1995; 67:466–471.

    Article  Google Scholar 

  91. Rigby GP, Ahmed S, Horseman G, Vadgama P. In vitro glucose monitoring with open microflow — influences of fluid composition and preliminary evaluation in man. Analytica Chimica Acta 1999; 74:23–32.

    Article  Google Scholar 

  92. Kros A, Gerritsen M, Sprakel VSL, Sommerdijk NAJM, Jansen JA, Nolte RJM. Silica-based hybrid materials as biocompatible coating for glucose sensors. Sensors and Actuators B 2001; 81:68–75

    Article  Google Scholar 

  93. Schmidtke DW, Heller A. Accuracy of the one-point in vivo calibration of ‘wired’ glucose oxidase electrodes implanted in jugular veins of rats in periods if rapid rise and decline of the glucose concentration. Analytical Chemistry 1998; 70:2149–2155.

    Article  Google Scholar 

  94. Frost MC, Batchelor MM, Lee Y, Zhang H, Kang Y, Oh B, et al. Preparation and characterisation of implantable sensors with nitric oxide realise coating. Microchemical Journal 2003; 74:277–288.

    Google Scholar 

  95. Schoenfisch MH, Mowery KA, Rader MV, Baliga N, Wahr JA, Meyerhoff ME. Improving the thromboresitivity of chemical sensors via nitric oxide release: fabrication and in vivo evaluation of NO-releasing oxygen-sensing catheters. Analytical Chemistry 2000; 72:1119–1126.

    Article  Google Scholar 

  96. Klueh U, Dorsky DI, Kreutzer DL. Enhancement of implantable glucose sensor function in vivo using gene transfer-induced neovascularization. Biomaterials 2005; 26:1155–1163.

    Article  Google Scholar 

  97. Updike SJ, Shults MC, B.J. Gilligan, R.K. Rhodes. A subcutaneous glucose sensor with improved longevity, dynamic range, and stability of calibration. Diabetes Care 2000; 23:208–214.

    Article  Google Scholar 

  98. Newman JD, Turner APF. Home blood glucose biosensors: a commercial perspective. Biosensors and Bioelectronics 2005; 20:2435–2453.

    Article  Google Scholar 

Download references

Author information

Authors and Affiliations

  1. Imperial College London, UK

    Anna Radomska, Suket Singhal & Tony Cass

Authors
  1. Anna Radomska
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Suket Singhal
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Tony Cass
    View author publications

    You can also search for this author in PubMed Google Scholar

Editor information

Editors and Affiliations

  1. Institute of Biomedical Engineering and Department of Computing, Imperial College London, UK

    Guang-Zhong Yang PhD

Rights and permissions

Reprints and permissions

Copyright information

© 2006 Springer-Verlag London Limited

About this chapter

Cite this chapter

Radomska, A., Singhal, S., Cass, T. (2006). Protein Engineering for Biosensors. In: Yang, GZ. (eds) Body Sensor Networks. Springer, London. https://doi.org/10.1007/1-84628-484-8_3

Download citation

  • DOI: https://doi.org/10.1007/1-84628-484-8_3

  • Publisher Name: Springer, London

  • Print ISBN: 978-1-84628-272-0

  • Online ISBN: 978-1-84628-484-7

  • eBook Packages: Computer ScienceComputer Science (R0)

Publish with us

Policies and ethics

Search

Navigation