This study systematically investigates the affinities of mononucleotides, i.e., adenine (A), thymine (T), guanine (G), and cytosine (C), and polynucleotides (20 bases, Poly A, Poly T, Poly G, Poly C) for lead sulfide (PbS) nanoparticles by evaluating the stability of mononucleotide- and polynucleotide-modified PbS nanoparticles in aqueous and salt solutions over time. Experimental results reveal distinct affinity trends between mononucleotides and polynucleotides. For mononucleotides, the affinity order is A>G>C>T, with C and T failing to stabilize PbS nanoparticles due to weak interactions. In contrast, the affinity order of polynucleotides differs from that of mononucleotides: Poly C>Poly G>Poly T>Poly A, indicating no direct correlation between the two systems. The discrepancy is attributed to the rigid length of polynucleotides, a critical factor reflecting the rigidity of oligonucleotide chains. Shorter rigid lengths enable more contact points between polynucleotides and the PbS surface, enhancing stability through cumulative interactions, even when individual nucleotide affinities are weak (e.g., Poly T). For example, Poly A's longer rigid length limits contact points, reducing stability despite A's strong single-nucleotide affinity. Transmission electron microscopy (TEM) confirms that Poly A-modified PbS nanoparticles (average size of 9.4 nm) exhibit good dispersibility and crystallinity, while high-concentration Poly G assemblies induce aggregation via intermolecular G-wire formation. These findings highlight the necessity of considering both nucleotide affinity and chain rigidity when designing oligonucleotide-stabilized semiconductor nanoparticles, thus providing a foundation for DNA-guided synthesis of PbS nanoparticles with tailored properties for applications in bioimaging and optoelectronics.