An inexpensive detector for proteins is described. The detection technique was based on two-photon excitation intrinsic protein fluorescence using a visible 532 nm diode-pumped nano laser as the excitation source. Proteins that exhibit intrinsic fluorescence must contain at least one tryptophan, tyrosine, or phenylalanine residue in their amino acid sequences. The detector was characterized and was found to have a detection limit of 4 micro-molar for tryptophan, 22 micro-molar for tyrosine and 500 micro-molar for phenylalanine. Bovine serum albumin, a serum protein with 3 tryptophan residues in its amino acid sequence was also used to characterize the detector. It was found that the detection limit for this protein was 0.9 micro-molar. The detector volume was determined based on a photon counting histogram - a technique in fluorescence fluctuation spectroscopy. From the results of this analysis, the excitation volume was found to be 2.9 fL. With such an excitation volume, the detection limits were either within or below the atto-mole range.