In order to conjugate protein to nanopartcicle you should use chemical cross-linking. There are a lot of chemistries for cross-linking. One of the popular is a CNBr derivative, commonly used to couple proteins and beads in affinity chromatography. The successful conjugation can be monitored and the conjugate can be separated by use of size-exclusion cromatography on suitable resin (Superose 6 or Sephacryl S 500 for example)
first you have to carefully study the structure of the protein and and to ensure that nano particle functionality is good before selecting suitable linkers, of which the list is endless.
From my experiences: I used magnetsensitive aminofuctionalized silica nanocomposites. Nucleus - Fe3O4, shell - aminopropilsilocsan, bis-3, 3 metiloxisililpropilamin and other with aminogroups. Proteins - Ig(s). Glutaraldegid was used for conjugation of nanoparticles and protein. Conjugat are removed by magnet. Concentation of protein is detected (method Lowry or other) before conjugation, in the course of time (30min, 60 min....) and in the finish (in volum). It will be kinetic of connection.
Tiofunctionalized nanoparticles are conjugated by maleimidobenzoils, for example 4-(N-maleimidomethyl)cyclohexanecarboxylic acid N-hydroxysuccinimide ester (Sigma M5525) and other, look catalog, www.sigma-aldrich.com.