Graphene oxide nanosheets (GONs) have attracted considerable interest due to their potential applications in electronic and optoelectronic devices. Graphene oxide (GO) is usually prepared by using Hummers' method or modified Hummers' methods, which require graphite and strong oxidizers, followed by exfoliation to obtain GONs. The main drawbacks of this top-down approach are the use of strong oxidizing agents and small lateral size of GONs. Here, we present a self-assembly method to synthesize GONs with tunable thickness ranging from 1 nm (monolayer) to 1500 nm. The lateral sizes of the monolayer and few-layer (<5) GONs are about 20 μm and 100 μm respectively. The GONs are prepared by a hydrothermal method using glucose as a sole reagent. The method is environmentally friendly, facile, low-cost as well as capable of scaling up for mass production. The electrical, optical and structural properties of the as-grown and annealed GONs are comparable to GO and reduced GO prepared by the top-down method, respectively. The electrical resistivity of the GONs can be tuned by annealing for 8 orders of magnitude ranging from 10⁶ Ω cm to 10⁻² Ω cm. A GO-based photodetector has been fabricated, demonstrating the optoelectronic properties of the GONs.