Fluorescence Detection 400–480？nm Using Microfluidic System Integrated GaP Photodiodes

Ciprofloxacin is a commonly used antibiotic and the active ingredient in a veterinary antibiotic. Detecting its presence allows us to understand its absorption process in blood as well as tissue. A portable microfluidic system has been fabricated. It operates at low bias voltage and shows a linear relationship between concentration levels and system response. Detection of concentrations down to 1？ppb of ciprofloxacin in microliters of solution was achieved. 1. Introduction As the medical industry has evolved, it has become necessary to have accurate testing available in the shortest possible time [1–5]. Miniaturizing UV and fluorescence detectors and extending them to fluid analysis is one way of bridging this gap [6–9]. Detecting ciprofloxacin has become increasingly important because it is the active ingredient in enrofloxacin an antibiotic used in veterinary medicine [10, 11]. Interest in monitoring ciprofloxacin levels in the tissues of animals raised for food and in the milk of cows has led to improvements in current technology and a search for new detection techniques. Ciprofloxacin absorbs strongly in the UV (≤280？nm) and emits at ~ 440？nm [10]. Currently, online detection is carried out by analytical methods such as high-performance liquid chromatography (HPLC), which involves a relatively costly investment in hardware [12, 13], or by capillary electrophoresis. Capillary electrophoresis (CE), while simpler than the aforementioned HPLC, still requires complex sample preparation methods utilizing pH adjustment to reach lower detection limits and requires very high bias voltage (kV) [12, 14]. These two methods usually employ photomultiplier tubes (PMTs) for photodetection. PMTs have been used in applications such as low-level ultraviolet detection in laser-induced fluorescence biological-agent warning systems [15]. Other applications in this wavelength range are under-water detection at 400？nm, the wavelength at which water is transparent, and detection of 440？nm-wavelength light from scintillation crystals that are used to sense gamma rays from nuclear material. While PMTs are among the most sensitive detectors currently available, semiconductor photodiodes offer the advantages of being less expensive and more robust. Si photodiodes have high responsivities at 440？nm; however, GaP exhibits a detection cutoff wavelength of 550？nm, which makes it an attractive alternative to Si, which requires expensive filters to reject extraneous wavelengths. This paper expands on our previous success in integrating photodiodes with microfluidics for ciprofloxacin