An scFv peptide comprising a VH domain and a VL domain linked by an amino acid spacer is disclosed. The VH domain comprises a sequence with at least 80% sequence identity to the sequence of SEQ ID NO. 64. The VL domain comprises a sequence with at least 80% sequence identity to the sequence of SEQ ID NO. 66. The scFv peptide also comprises the substitution or deletion of an amino acid in the VH domain at the position corresponding to C28.