This paper presents a cost-effective, ultra-low-power and highly sensitive wake-up receiver (WuRX) to reduce overall power consumption of Wireless Sensor Networks (WSN). The proposed tuned radio frequency (TRF) receiver (RX) analogue front-end (AFE) incorporates a 400-MHz, low-power, low-noise amplifier (LNA) with a high voltage gain of 50 dB as well as a Gilbert Cell based envelop detector achieving a conversion gain of 224V-1 to enhance the overall sensitivity of the WuRX. Meanwhile, a duty-cycling technique is used to reduce the power consumption of the WuRX AFE. In addition, a 31-bit correlator is implemented in the WuRX digital back-end (DBE) to further improve the sensitivity of the WuRX by 4 dB. A proof-of-concept WuRX circuit prototype has been fabricated in a low-cost 180-nm CMOS technology. Measurement results show the complete WuRX attains a sensitivity of -84.5 dBm at a wake-up error rate (WER) of 10-3 whilst only consuming 193nW at the minimum data rate of 64 bps with a carrier frequency of 402 MHz. With the sensitivity unchanged, the data rate of the WuRX can be scaled up from 64 bps to 8.2 kbps. The used 4-time oversampling technique makes the WuRX robust against asynchronization and clock deviation between the TX and RX. The proposed WuRX is a promising solution for energy-efficient rendezvous between wireless sensor nodes, particularly in application scenarios where both low power consumption and high sensitivity are indispensable.