## 1 Introduction  
Graphene, a unique type of two-dimensional semiconductor with superior electronic, thermal, and mechanical properties and chemical stability, has been applied in diverse fields such as electronics, magnetics, and optics [1; 2; 3; 4]. Graphene quantum dots (GQDs), a new kind of luminescent zero-dimensional carbon materials with finite size, have attracted tremendous interest from scientists. Because of the quantum confinement in GQDs, it is expected that they will yield many interesting phenomena which do not exist in other semiconductor materials [5; 6]. Compared with traditional QDs, GQDs show many advantages, e.g., low toxicity, low-cost synthesizing strategies, good solubility in water, and organic solvents [7]. Therefore, GQDs are considered as promising candidates to replace traditional QDs, and have been integrated into many fields such as bioimaging [8; 9], sensing [10], catalysts [11; 12; 13], and photovoltaic devices [14].  
Nowadays, many methods have been developed to prepare GQDs using large graphene sheets as raw materials through physical and chemical methods, such as hydrothermal and solvent thermal treatments, ultrasonic shearing, electrochemical oxidation, and oxygen plasma treating [15; 16; 17; 18; 19; 20; 21]. [PERSON] et al. [15; 16] synthesized strong blue and green luminescent GQDs by cutting graphene sheets through the hydrothermal route. Concentrated H\({}_{2}\)SO\({}_{4}\) and HNO\({}_{3}\) were used to oxidize the graphene sheets. [PERSON] et al. [17] obtained GQDs by photo-Fenton reaction of graphene oxide (GO) under the UV irradiation, the GQDs have uniform crystallinity, and present a strong photoluminescence property. [PERSON] et al. [19] reported an electrochemical approach with a filtration-formed graphene film as a working electrode in PBS for direct preparation of functional GQDs, which exhibited green luminescence. [PERSON] et al. [22] prepared GQDs from plant leaves through a green one-pot hydrothermal method; GO formed after 2 h and GQDs formed after 8 h of the hydrothermal reaction. The obtained GQDs displayed intense green photoluminescence (PL). Our previous work successfully synthesized different colored GQDs by cutting oxidized GO through a hydrothermal method; the oxidized GO was obtained by oxidation of GO with O\({}_{3}\)[23]. Oxidation is an important part in synthesizing GQDs using graphene, GO, and CNT as starting materials, because it can introduce oxygen-containing groups or defects onto the surface or edge of the carbon materials, and increase active sites. Generally, oxidants used in oxidation include H\({}_{2}\)SO\({}_{4}\)-KMnO\({}_{4}\), concentrated H\({}_{2}\)SO\({}_{4}\) and HNO\({}_{3}\), Fenton reagent, O\({}_{3}\), and so on [15; 17; 24; 25; 26].  
Some of the techniques mentioned above are complex and expensive. [PERSON] et.al. [27] prepared single-layered GQDs using SWCNT as a precursor through a simple and green, three-step hydrothermal etching method. They chose different concentrations of HNO\({}_{3}\) as oxidant; the as-prepared GQDs present green and yellow luminescence. Concentrated HNO\({}_{3}\) is excellent in oxidation, introducing nirtrogen (N) and oxygen-containing groups into GQDs.  
GO, with oxygen-containing groups on the surface and edge has similar sp\({}^{2}\) carbon networks to SWCNT, while it has a layered structure; therefore, it is predicted that GO should be an ideal raw material for preparing GQDs, adopting concentrated HNO\({}_{3}\) as oxidant. However, to the best of our knowledge, no attention has been paid to cutting GO into GQDs by using HNO\({}_{3}\) as oxidant.  
In this work, we present a simple, three-step hydrothermal method to synthesize fluorescent GQDs by using GO as raw material and HNO\({}_{3}\) as the oxidant. Two kinds of GQDs with different degrees of oxidation and N doping have been obtained. The mechanism for the formation of GQDs and their fluorescence properties have been discussed in detail.