Abstract: Micron and submicron fiber-optic chemical and biochemical sensors have been developed recently in our laboratory. Here we give a theoretical discussion, based on pragmatic modes of operation, of the advantages and disadvantages of optode miniaturization. In most instances we present explicit functional dependences of specific optode characteristics on the optode radius (r). For instance, the absolute detection limit decreases with r$+3$/ (good!), the response time reduces with r$+2$/ (good!), while the signal to noise ratio decreases with r (bad!) but not r$+3$/ (luckily!) under our standard working conditions. Other features that improve with down-sizing include sample volume, sensitivity, invasiveness, spatial resolution, enzyme activity, heating of sensor and/or sample, toxicity and materials cost. Features that may worsen include fluorophore leaching and photodamage to sensor and/or sample. Methods for overcoming these disadvantages make use of the shorter response times of small optodes and forward optical signal collection with small samples (utilizing standard lab microscopes). We refer to the ultimate goal of non-destructive single molecule detection and imaging. !14
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