Detection of low-abundance nucleic acids is a challenging task, which over the last two decades has been solved using enzymatic target amplification. Enzymatic synthesis enhances the signal so that diverse, scientifically and clinically relevant molecules can be identified and studied, including cancer DNA, viral nucleic acids, and regulatory RNAs. However, using enzymes increases the detection time and cost, not to mention the high risk of mistakes with amplification and data alignment. These limitations have stimulated a growing interest in enzyme-free methods within researchers and industry. In this review we discuss recent advances in signal-enhancing approaches aimed at nucleic acid diagnostics that do not require target amplification. Regardless of enzyme usage, signal enhancement is crucial for the reliable detection of nucleic acids at low concentrations. We pay special attention to novel nanomaterials, fluorescence microscopy, and technical advances in detectors for optical assessment. We summarize sensitivity parameters of the currently available assays and devices which makes this review relevant to the broad spectrum of researchers working in fields from biophysics, to engineering, to synthetic biology and bioorganic chemistry.