pH-and oxygen sensors based on fluorescent nanoparticles for Lab-on-Chip applications

Introduction We present the synthesis of silica (SiO2)-nanoparticles functionalized with fluorescent dyes. These nanoparticles were developed for multi-sensor spots directly inkjet-printed on microfluidic sytems such as Lab-on-Chip (LoC). The nanoparticle-based sensor spots have well-defined fluorescent properties and can be modified for sensing different environmental parameters. This was demonstrated by developing fluorescent nanoparticles whose fluorescent intensity strongly depends on the pH-value of the solution. Methods We used the Stöber method (sol-gel process) to synthezise SiO2-nanoparticles. The nanoparticles were functionalized by either a pH-sensitive (Fluorescein isothiocyanate (FITC)) or a dissolved oxygen (DO)-sensitive (Tris(2,2-bipyridyl)dichlororuthenium(II)hexahydrate ([Ru(bpy)3]2+)) dye. The distribution of nanoparticle size was evaluated by scanning electron microscope (SEM) and dynamic light scattering. The fluorescenct properties (excitation and emission spectrum) of nanoparticles and the pH/oxygen-dependency of the fluorescent intensity were measured using a microplate reader. Results We obtained spherical SiO2 nanoparticles with a high monodispersity (typical geometric standard deviation of 1.1). Using the Stöber process it is possible to synthesize SiO2 nanoparticles with adjustable diameters between 40 nm and 3.6 mm. Fluorescence measurements showed that the functionalization was successful with both the pH- and the oxygen-sensitive dye. The nanoparticles functionalized with FITC showed a linear dependence of fluorescence intensity in the pH range from 4.7 to 8.8 with increasing pH values. The relative increase in intensity was 22 % per pH change relative to the maximum intensity value. Conclusion We successfully established a procedure for synthetizing and functionalizing SiO2-nanoparticles of defined size. The nanoparticles can be functionalized by dyes whose fluorescent properties change with altered environmental conditions such as temperature, pH, DO, Ca2+ etc. Such nanoparticles are of particular interest as an ink for printing fluorescent multi-sensor spots in lab-on-chip applications.