The invention provides a hapten comprising a 6-[D-α-aminoacetamido] penicillin derivative crosslinked at the α-amino group with a substituted or unsubstituted phenyldicarbaldehyde. In addition, the invention provides an immunogen comprising the aforementioned hapten coupled to an antigenicity-conferring carrier material, a conjugate comprising the aforementioned hapten coupled to a labelling agent, as well as, antibodies raised against the aforementioned immunogen and capable of binding with at least one structural epitope of an intact β-lactam ring.