photophysical properties that are potentially useful for different applications. [1-5] One of the many fascinating properties of CDs is its fluorescence. Many reports have shown that the fluorescence of CDs originates from the surface fluorophore and graphitizing core. [6-9] However, the transition between singlet and triplet states occurs with extremely low probability due to the spin-forbidden, hence the room temperature phosphorescence (RTP, T 1 →S 0) and thermally activated delayed fluorescence (TADF, transitions T 1 →S 1 →S 0) are difficult to be achieved under ambient conditions. [10-12] Other than that, afterglow from pure CDs is seldom observed and studied. [13-17] A selfquenching resistance CDs with RTP lifetime of 13.4 ms were induced by polyvinyl alcohol (PVA) chains from Liu's group. [13] Through seeded growth method, Andrey et al. prepared several phosphorescent CDs powders with average decay lifetime ranging from 52 to 419 ms. [14] Yang's group focused on suppressing the nonradiative transitions having achieved the adjustable decays between 188 and 658 ms. [15] Feng and co-workers found that fluorine and nitrogen codoped CDs with RTP could realize the decay lifetime of 141 to 1210 ms by adjusting pH values. [16] It is noteworthy that long-lived emission (LLE) about 1.46 s of CDs powder was successfully achieved via microwave irradiation but with very poor ability to resist moisture that RTP totally disappeared in solution and also quite faint emission intensity only with quantum yield (QY) of 3.53% by Lin et al. [17] As for these pure CDs, phosphorescence that is responsible for the total afterglow without TADF component only lasts for a few seconds accompanied by a relatively low phosphorescent efficiency. Furthermore, these pure CDs have extremely poor stability, thus afterglow of them was completely quenched by oxygen or moisture, causing the RTP to be only captured in powdery CDs rather than solution. [13-17] Nevertheless, a great deal of effort was devoted to improving the afterglow behavior. In some cases, the RTP can be ameliorated by introducing heavy atoms, such as halogens and metals which enhanced the intersystem crossing (ISC) process attributed to the strong spin-orbit coupling. [18,19] Alternatively, embedding CDs in solid matrices that serve Carbon nanodots (CDs) anchored onto inorganic supporter (amorphous nanosilica, SiO 2) like a core-satellite structure have enhanced the room-temperature phosphorescence (RTP) intensity along with ultralong lifetime of 1.76 s. Special and quite stable structure should account for these superiorities, including hydrogen network, covalent bond, and trap-stabilized triplet-state excitons that are responsible for the generation of phosphorescence. These multiple effects have efficaciously protected CDs from being restrained by the external environment, providing such long-lived emission (LLE) that can subsist not only in powdery CDs-SiO 2 but also coexist in aqueous solution, pushing a big step forward in the application prospects of liquid-state phosp...

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Objective. Steady-state visual evoked potential (SSVEP) is an essential paradigm of electroencephalogram based brain–computer interface (BCI). Previous studies in the BCI research field mostly focused on enhancing classification accuracy and reducing stimuli duration. This study, however, concentrated on increasing the number of available targets in the BCI systems without calibration. Approach. Motivated by the idea of multiple frequency sequential coding, we developed a calibration-free SSVEP–BCI system implementing 160 targets by four continuous sinusoidal stimuli that lasted four seconds in total. Taking advantage of the benchmark dataset of SSVEP–BCI, this study optimized an arrangement of stimuli sequences, maximizing the response distance between different stimuli. We proposed an effective classification algorithm based on filter bank canonical correlation analysis. To evaluate the performance of this system, we conducted offline and online experiments using cue-guided selection tasks. Eight subjects participated in the offline experiments, and 12 subjects participated in the online experiments with real-time feedbacks. Main results. Offline experiments indicated the feasibility of the stimulation selection and detection algorithms. Furthermore, the online system achieved an average accuracy of 87.16 ± 11.46% and an information transfer rate of 78.84 ± 15.59 bits min−1. Specifically, seven of 12 subjects accomplished online experiments with accuracy higher than 90%. This study proposed an intact solution of applying numerous targets to SSVEP-based BCIs. Results of experiments confirmed the utility and efficiency of the system. Significance. This study firstly provides a calibration-free SSVEP–BCI speller system that enables more than 100 commands. This system could significantly expand the application scenario of SSVEP-based BCI. Meanwhile, the design criterion can hopefully enhance the overall performance of the BCI system. The demo video can be found in the supplementary material available online at stacks.iop.org/JNE/18/046094/mmedia.

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