Disclosed are modified cytosine bases that provide enhanced base-pairing affinity for guanine bases in polynucleotide hybridization complexes. Also disclosed are polynucleotide oligomers, polynucleotide hybridization complexes that comprise such modified cytosine bases. Also disclosed are various methods of use. For example, in some embodiments, modified polynucleotide oligomers disclosed herein can be used as primers and probes for nucleic acid amplification and/or detection.