Optical antennas represent an enabling technology for enhancing the detection of molecular vibrational signatures at low concentrations and probing the chemical composition of a sample in order to identify target molecules. However, efficiently detecting different vibrational modes to determine the presence (or the absence) of a molecular species requires a multispectral interrogation in a window of several micrometers, as many molecules present informative fingerprint spectra in the mid-infrared between 2.5 and 10 μm. As most nanoantennas exhibit a narrow-band response because of their dipolar nature, they are not suitable for such applications. Here, we propose the use of multifrequency optical antennas designed for operating with a bandwidth of several octaves. We demonstrate that surface-enhanced infrared absorption gains in the order of 10(5) can be easily obtained in a spectral window of 3 μm with attomolar concentrations of molecules, providing new opportunities for ultrasensitive broadband detection of molecular species via vibrational spectroscopy techniques.