There are several methods you can use to determine the presence of carboxylic acid (COOH) groups on a particle surface. Here are a few common techniques:

1. FTIR Spectroscopy: Fourier-transform infrared (FTIR) spectroscopy can be used to identify the presence of carboxylic acid groups based on characteristic peaks in the FTIR spectrum. The carboxylic acid functional group typically exhibits a strong absorption band in the range of 1700-1750 cm^-1. By comparing the FTIR spectra of your modified particles to those of unmodified particles, you can assess the presence of COOH groups.

2. X-ray Photoelectron Spectroscopy (XPS): XPS is a surface-sensitive technique that can provide information about the chemical composition of the outermost layer of your particles. It can distinguish between different chemical states of carbon, including COOH groups. By analyzing the high-resolution carbon XPS spectra, you can determine the concentration of COOH groups on the surface.

3. Titration Methods: You can perform acid-base titrations to quantify the number of COOH groups on your particle surface. This involves titrating the surface with a known concentration of a base (e.g., sodium hydroxide) and monitoring the pH change. The change in pH can be used to calculate the concentration of COOH groups.

4. Zeta Potential Measurements: Zeta potential measurements can provide information about the surface charge of particles in solution. The modification of hydroxyl groups to COOH groups may change the surface charge, which can be detected by zeta potential analysis. An increase in the negative zeta potential may indicate the presence of COOH groups.

5. Nuclear Magnetic Resonance (NMR): Solid-state NMR can be used to analyze the chemical structure of the particle surface. By comparing NMR spectra of modified and unmodified particles, you can detect the presence of COOH groups.

6. Chemical Assays: There are specific chemical assays available for quantifying carboxylic acid groups. One example is the use of a carboxylic acid assay kit, which can provide a quantitative measurement of COOH groups based on a colorimetric reaction.

Keep in mind that the choice of method may depend on factors such as the nature of your particles, the extent of modification, and the level of sensitivity required. It's often a good practice to use multiple complementary techniques to confirm the presence of COOH groups on your particle surface and to quantify them accurately.

For something as large as a particle, of which I assume you are probably on a 10-micron scale or thereabout, your best bet is to contact Agilent Technologies, who perform that exact type of measurement every day. Agilent makes HPLC columns, among other things. The HPLC columns are these days on a 3-micron particle scale. They typically attach functional groups onto silica and then have to assay that to determine how efficient the chemistry was.

Agilent is at 800-227-9770, ask for HPLC column tech support and tell them what you are doing. They are the leading experts in this.

All of the methods 1 through 6 in the prior message by Swu cannot be done on particle surfaces, they are incapable of that type of sensitivity.