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The sensor exploits the phenomenon of upconversion luminescence and is based on (a) the use of upconverting nanoparticles (UCNPs) of the NaYF(4):Yb,Er type that can be excited with 980 nm laser light to give a green and red luminescence and (b) the pH probe phenol red immobilized in a polystyrene matrix. Exposure of the sensor film to ammonia causes a strong increase in the 560 nm absorption of the pH probe which, in turn, causes the green emission of the UCNPs to be screened off. The red emission of the UCNPs, in contrast, remains unaffected by ammonia and can serve as a reference signal. Due to the use of 980 nm as the excitation light source, the optical signal obtained is completely free of background visible luminescence of the sample and of scattered light. This is highly advantageous in the case of sensing ammonia in complex matrixes.

We report on the preparation of fluorescent silica nanoparticles (NPs). They have been prepared by (a) modification of the NPs by amino groups and subsequent introduction of amino-reactive fluorophores of various color and (b) by modification of the NPs by either azido groups or alkyne groups and subsequent conjugation to fluorophores by so-called click chemistry, which is a novel approach toward modifying silica NPs. The new NPs were characterized in terms of size and spectral properties.

A method is described for modifying the surface of upconverting microparticles (UCmuPs) and nanoparticles (UCNPs) such that they become amenable to click chemistry. Respective reagents are presented and used in both kinds of particles, either directly or in combination with tetraethoxysilane. The particles also were labeled by using the click reaction, a) with fluorophores to yield materials that have emission colors that depend on the wavelength of excitation; b) with maleinimido groups (so to obtain labels for thiols), and c) with biotin (to make them useful for affinity studies based on the biotin-streptavidin system). We believe that both the UCmuPs and UCNPs have the potential of being used in numerous areas including upconversion imaging, biolabeling, and derivatization, but also in encoding and security.