<p>An ultrahigh-intensity femtosecond laser can establish a longitudinal electric field stronger than 10¹³ Vm⁻¹ within a plasma, accelerating particles potentially to GeV over a sub-millimetre distance. Laser-accelerated protons with high brightness and picosecond duration are highly desired for applications including proton imaging and flash radiotherapy, while a major limitation is the relatively low proton energy achieved yet, primarily due to the lack of a controllable acceleration structure. Here, we report the generation of protons with a cutoff energy exceeding 110 MeV, achieved by irradiating a multi-petawatt femtosecond laser on a conduction-restricted nanometre polymer foil with a finite lateral size. The enduring obstacles in achieving ultrahigh laser contrast and excellent laser pointing accuracy were successfully overcome, allowing the effective utilization of size-reduced nanometre foils. A long acceleration structure could be maintained in such a quasi-isolated foil since the conduction of cold electrons was restricted and a strong Coulomb field was established by carbon ions. Our achievement paves the road to enhance proton energy further, well meeting the requirements for applications, through a controllable acceleration process using well-designed nano- or micro-structured targets.</p>