The ability to detect nanoscale objects is particular crucial for a wide range of applications, such as environmental protection, early-stage disease diagnosis and drug discovery. Photonic crystal nanobeam cavity (PCNC) sensors have attracted great attention due to high-quality factors and small-mode volumes (Q/V) and good on-chip integrability with optical waveguides/circuits. In this review, we focus on nanoscale optical sensing based on PCNC sensors, including ultrahigh figure of merit (FOM) sensing, single nanoparticle trapping, label-free molecule detection and an integrated sensor array for multiplexed sensing. We believe that the PCNC sensors featuring ultracompact footprint, high monolithic integration capability, fast response and ultrahigh sensitivity sensing ability, etc., will provide a promising platform for further developing lab-on-a-chip devices for biosensing and other functionalities.Micromachines 2020, 11, 72 2 of 20 microcavities confine the light in a small volume, leading to significant enhancement of light-matter interaction, resulting in ultra-high sensitivity (S) and a low detection limit. When subject to slight environmental changes, the spectral properties change can be obtained, e.g., resonator wavelength shift [20][21][22] and mode broadening [23].The main types of optical microcavities include whispering gallery mode (WGM) cavities, Fabry-Perot (F-P) cavities and photonic crystal (PhC) cavities [24]. Among these, PhC cavities have been investigated as advantageous platforms due to their ultra-high Q/V enabling the enhancement of lightmatter interactions. Particularly, compared with two-dimensional (2D) PhC cavities, one-dimensional photonic crystal nanobeam cavities (1D-PCNCs) attract great attention for their ultra-sensitive optical sensing and lab-on-a-chip applications, owing to their ultra-compact size, ultra-small mode volume, high integrability with bus-waveguide and excellent complementary metal-oxide-semiconductor (CMOS) compatible properties [25][26][27][28][29][30][31][32][33].In this review, we will focus on nanoscale optical sensing based on 1D-PCNCs. The structure of the review is organized as follows. A comprehensive overview about basic properties and sensing mechanisms of 1D-PCNCs sensors is discussed in Section 2. In Section 3, firstly, we introduce the efforts to pursue ultra-high figure of merit (FOM) in refractive index (RI) sensing based on 1D-PCNCs. Secondly, we outline the applications of 1D-PCNC sensors on single nanoparticle and label-free molecule (viruses, proteins and DNAs) detection. Thirdly, a monolithic integrated 1D-PCNC sensor array for multiplexed sensing is introduced explicitly. In addition, we present other sensing applications of 1D-PCNC sensors such as temperature sensing and optomechanical sensing, etc. Next, we review the nanofabrication and coupling techniques in the development of 1D-PCNC sensors in Section 4. Finally, we draw a brief conclusion. Figure 2. (a) Schematic of photonic circuits. The green lines, red boxes and black boxes represe...