Measuring pH at the nanoscale is important in many areas, such as intracellular pH and microfluidic device pH tracker. The conventional pH measurement method, such as pH meter and pH paper, is not easy to fit into the nanoscale device. People usually use fluorescence dye to measure the pH at the nanoscale. For example, fluorescein is commonly used pH sensitive dye by measuring its fluorescence intensity to detect the pH. Carboxy-SNARF-1 (SNARF) is another pH sensitive fluorescence dye by detecting its ratio of fluorescence. The fluorescence dye is small, mobile and can be attachable to living organisms and molecule. However, the emission of dye solution can be quenched by environment, the diluted dyes have weak signal for detection and it is susceptible to electrical or chemical damage.

In this project we have made calcium phosphate nanoshells with the encapsulation of fluorescence dye for pH measurement. The nanoshell is a self-contained nanosized device, which can enter into nanoscale places. It can protect the pH sensitive fluorescence dye from chemical and electrical damage. The nanoshell can also reduce the light flickering and stronger pointed light source visible with a regular fluorescent microscope. Calcium phosphate coating on the nanoshells is biocompatible, so the nanoshells is suited for in vitro/in vivo use. The nanoshells can also be functionalizable with antibodies for targeted deliver. The nanorshells are synthesized by preparing Egg PC liposomes in size with diameter of 100 nm in phosphate buffer and titrating with calcium chloride to form a hard shell of calcium phosphate. Nanoshells were characterized by TEM and DLS. Fluorescein and SNARF dye was encapsulated inside the nanoshell respectively for measuring the pH. The emission spectra of fluorescein and SNARF in solution and in nanoshells for different pH were obtained. The plot of emission intensity versus pH for fluorescein in nanoshells shows the same trend as the plot for fluorescein in solution. In addition, the plot of emission ratio versus pH for SNARF in nanoshells and in solution also shows the same trend. These results suggest that calcium phosphate nanoshells with pH sensitive fluorescence dye can be used to measure the pH at nanoscale.