The authors performed scanning tunneling spectroscopy (STS) analysis of single-strand deoxyribonucleic acid (DNA) with fluorescein isothiocyanate (FITC) dye at the 5' terminal (FITC-5'-agctgtac-3'). They obtained high-resolution topographic scanning tunneling microscope images of the DNA that showed eight nucleotide bases and one FITC molecule. The current versus voltage (I-V) data of FITC had local maxima or plateaus, whereas those of DNA had simple growth curves. They also found that in the STS analysis, the I-V data of the DNA were fitted well by an exponential curve in the positive and negative bias regions. They also discuss the possibility of whether four nucleotides (adenine, guanine, cytosine, and thymine) can be differentiated through STS experiments. (c) 2007 American Vacuum Society.