The development of optical refractive index sensors for label-free sensing is beneficial for both chemical and biochemical applications. Lots of efforts have been devoted to narrow the resonance peaks of periodic nanostructures and, therefore, improve the figures of merit. The substrates with high-quality factor resonances always come at the expense of not only complicated fabrication processes but also the requirement of sophisticated optical measuring systems. It is demonstrated in this work that Fabry-Perot resonance based broadband sensing with figure of merit of 83 can be achieved using low-cost self-assembled opal photonic crystals. It is seen by the naked eye that the transparency of photonic crystal dots can be gradually improved by increasing the refractive index of the filling liquid. The loop-mediated isothermal amplification induced refractive index variation of biological samples has also been recognized using the prepared photonic crystal dots, which are capable of fluorescence enhancement as well.