## 1 Introduction  
China, as one of the world's largest carbon emitters, has proposed the goal of carbon neutrality by 2060 [1]. The development of clean and renewable energy sources, such as wind power, solar power, hydropower, nuclear power, and biomass power, is an efficient solution to achieve this challenging goal [2, 3]. Among them, biomass, especially lignocellulosic, is an abundant, readily available, and sustainable resource. As is known, bamboo is a typical kind of lignocellulosic biomass with approximately 1500 species under 87 genera, and the total forest area of bamboo in China is approximately 6 million hectares [4, 5]. Notably, bamboo grows rapidly (a peak growth rate up to 100 cm per day) and can be harvested every 3-5 years, well exceeding the growth rate (typically 0.1-0.4 cm per day) and the 20-60-year growth cycle of traditional timber [6]. Consequently, bamboo is widely treated as a substitute for woody biomass, exerting enormous influence in easing the pressure on traditional timber [7]. However, the high-value utilization of lignocellulosic is generally hampered by its structural recalcitrance to a large extent. It is commonly believed that the recalcitrant and rigid characteristics of lignocellulosic are derived from its integral compact structure. To disrupt the compact structure of lignocellulosic, tremendous pretreatment techniques have been developed [8, 9, 10, 11]. Compared with other pretreatment techniques, hydrothermal pretreatment has various advantages such as high separation efficiency, high throughput, and the possibility of coordination with other pretreatment techniques [12]. Duringhydrothermal pretreatment, hydronium ions, generated from water auto-ionization, and carboxylic acids, liberated from deacetylation of hemicellulose, act as hydrolytic catalysts [13; 14]. Rather than cellulose, hemicellulose is much less resistant to hydrolysis [15]. [PERSON] et al. [16] showed that 80.2% of bamboo hemicellulose was dissolved during hydrothermal pretreatment at 180 \({}^{\circ}\)C for 90 min. [PERSON] et al. [17] studied the hydrothermal pretreatment of bamboo for xylo-oligosaccharide production, and their maximum recovery yield of xylo-oligosaccharides was 47.5% at 180 \({}^{\circ}\)C for 30 min. [PERSON] et al. [18] demonstrated that the highest xylo-oligosaccharide concentration of 10.5 g/L was obtained during hydrothermal pretreatment of bamboo at 180 \({}^{\circ}\)C for 40 min. However, due to the unintended hydrolysis of xylo-oligosaccharides, the yield of xylo-oligosaccharides from bamboo was only around 8.0% via hydrothermal pretreatment at 200 \({}^{\circ}\)C for 60 min [19]. It is generally acknowledged that lignin cannot be easily deconstructed using hydrothermal pretreatment [18], whereas deep eutectic solvent (DES) can easily dissolve lignin [20; 21]. DES also has promising properties such as non-toxicity, low volatility and flammability, biocompatibility, low cost, and ease of synthesis [22; 23; 24]. [PERSON] et al. [25] revealed that choline chloride (ChCl)-based DES was more efficient in delignification than betaine- or urea-based DES. [PERSON] et al. [26] pointed out that lactic acid (LA) could be manufactured via fermentation from agricultural products and exhibited high thermal stability with an evaporation temperature of \(\sim\)180 \({}^{\circ}\)C. [PERSON] et al. [27] investigated the effect of functional groups in carboxylic acid on lignin extraction, demonstrating that a short alkyl chain and an extra \(\alpha\)-hydroxyl group enhanced the extraction rate of lignin. Therefore, ChCl-LA DES is widely used in the delignification of lignocellulosic [28; 29; 30]. For example, [PERSON] et al. [28] demonstrated that the delignification of bamboo using ChCl-LA DES (80.1%) was much higher than that using betaine-LA DES (54.5%) at 140 \({}^{\circ}\)C for 6 h. Although ChCl-LA DES exhibits a superior performance in delignification as well as hemicellulose removal, hemicellulose was usually treated as waste in DES liquid [29; 30].