Search

Nitric oxide (NO) functions as an essential signalling molecule in various physiological and pathological pathways. In vitro and vivo redox processes mediated by reactive oxygen species (ROS) and nitric oxide (NO) directly influence the intracellular state. In this study, a red-emitting fluorescent nanoprobe, N,S-CDs@Zn-ICA, was synthesized to monitor NO fluctuations in living cells and zebrafish under the exposure to various pollutants. Red-emissive carbon dots (N,S-CDs) were synthesized by a hydrothermal method using o-phenylenediamine and urea as carbon / nitrogen sources, and H 2 SO 4 as sulfur source. Glutathione (GSH) was introduced to link N,S-CDs with metal organic complexes (Zn-ICA) through an amidation reaction to fabricate a carbon dot-based composite fluorescent probe, which greatly improved the selectivity, stability, and response time of the N,S-CDs. The composite probe has high selectivity and sensitivity with limit of detection (LOD) of 96.0 nM. Furthermore, the proposed probe was successfully used to monitor the dynamic changes in NO levels and evaluate oxidative stress in MCF-7 cells and zebrafish under the exposure to various pollutants, including seven heavy metal ions (such as Pb 2+ , Cd 2+ , and Hg 2+ ) and nine organic pollutants at different concentrations and exposure times. This work provides a novel strategy for constructing highly selective and red-emitting fluorescent probe for real-time and dynamic monitoring of NO and further evaluating oxidative stress induced by pollutants in vitro and in vivo via fluorescence imaging., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

On-site multi-pesticide residues detection is particularly urgent and challenging. Here, we fabricated an enzyme-free ratiometric fluorescent detection system in combination with a hinge-like dual-channel 3D microfluidic paper analytical device (3D μPAD) for simultaneous visual detection of carbaryl and glyphosate. Blue-emission 1-naphthol (Em. 470 nm) was hydrolyzed from carbaryl, while yellow-emission 2,3-diaminophenazine (Em. 570 nm) was produced with the aid of Cu 2+ for glyphosate sensing. Inner-filter effect between 1-naphthol or 2,3-diaminophenazine and green-emission carbon dots (Em. 510 nm) realized two ratiometric fluorescent detection systems. Remarkable color variation of green-blue for carbaryl (50.00-1100 μΜ) and yellow-green for glyphosate (5.00-600 μΜ) were observed on a dual-channel 3D μPAD without crosstalk. Their detection limits were 1.11 and 0.63 μΜ, respectively. The strategy realized simultaneous visual detection of carbaryl and glyphosate in food/herbal with excellent accuracy (spiked recoveries, 91.00-107.2%), high precision (RSD ≤ 8.43%), and superior selectivity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Tetracyclines residues, particularly oxytetracycline (OTC) and tetracycline (TC), have raised extensive concern because of their serious adverse effects on human health. Herein, a dual-response fluorescent probe based on nitrogen-doped carbon dots (N-CDs) and Eu 3+ hybrid (N-CDs-Eu 3+ ) was developed to selectively determine OTC and TC. The N-CDs act as ancillary ligands of Eu 3+ and recognition units of OTC/TC, while the Eu 3+ ions chelated with N-CDs can also specifically recognize OTC/TC. Upon inclusion of OTC/TC, an enhancement in Eu 3+ emission occurs due to the energy transfer from OTC/TC to Eu 3+ and the efficient elimination of quenching effect caused by H 2 O molecule, which is attributed to the incorporation of N-CDs; while the blue fluorescence emitted by the N-CDs decreases under the inner filter effect and static quenching effect caused by OTC/TC. Based on the double and reverse response signals, the ratiometric detection of OTC and TC in the range of 0.1-45 μΜ and 0.1-30 μΜ is achieved with a detection limit of 0.017 and 0.041 μM, respectively. In addition, the noticeable variation in fluorescence color of the probe is integrated with a smartphone-assisted analysis device for the rapid on-site quantitative assay of OTC, where the detection limit is 0.15 μΜ. The results show that this probe performs with excellent specificity and anti-interference for both OTC and TC, and satisfactory detection results are obtained in lake water, milk, and honey samples, thereby confirming that the probe exhibits promising application in food safety and environmental monitoring., Competing Interests: Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted., (Copyright © 2022. Published by Elsevier B.V.)

Cobalt as a transition metal ion is a biologically essential trace element, and plays an important role in various biological systems. The silicon nanoparticles (SiNPs) / gold nanoparticles (AuNPs) composite as a simple and efficient fluorescent probe was developed to detect Co 2+ and vitamin B 12 (VB 12 ) based on the selective aggregation and inner filter effect (IFE). The green-emitting SiNPs were synthesized by one-pot hydrothermal method, and the AuNPs were synthesized and modified with thioglycolic acid and cetyltrimethylammonium bromide. The fluorescent probe was fabricated by simple mixing the SiNPs and AuNPs together. In the presence of Co 2+ /VB 12 , AuNPs are selectively aggregated, which results in the enhancement of the local surface plasmon resonance absorption centered at 520 nm and the green fluorescence of SiNPs is significantly quenched via IFE. The fluorescence quenching efficiency of the probe is linearly proportional to the concentration of Co 2+ in the range of 0.1-80 µM with a low detection limit of 60 nM, which is far lower than the guideline value of Co 2+ in drinking water (1.7 µM). For vitamin B 12 (VB 12 ), its linear relationship is in the range of 0.1-100 µM, and the limit of detection is 69 nM. Furthermore, the proposed method shows good selectivity for the detection of Co 2+ and VB 12 , and does not need sophisticated pretreatment, only through simple filter. It has been applied in actual environmental water samples and drug tablets with satisfactory results., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Rapid, convenient, sensitive and simultaneous detection of distinct enzymes is urgently needed for diagnosis, therapeutics and prognostic of related diseases. Here, a new strategy for simultaneous monitoring γ-glutamyl transpeptidase (GGT) and alkaline phosphatase (ALP) activity has been fabricated based on dual-emission carbon dots (CDs). CDs were prepared by solvothermal treatment of Actinidia chinensis, which presents two fluorescent emissions at 471 nm (blue channel) and 671 nm (red channel). GGT and ALP activity can be detected based on inner filter effect (IFE) and static quenching effect (SQE) of blue and red channels of CDs, respectively. Linear ranges were 2.5-90 U L -1 and 5-200 U L -1 , and limit of detection (LOD) were 0.71 U L -1 and 1.2 U L -1 for GGT and ALP, respectively. Developed CDs can monitor GGT and ALP activity in human serum samples with satisfied recoveries (99.3%-108.6% for GGT, 98.4%-105.4% for ALP). Furthermore, the combination of CDs to sense GGT and ALP activity with OR logic gate can predict human health status. The design and application of dual-emission CDs can also be extended as promising tools to detect multianalytes using different channel signals., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Bacterial infection, especially multidrug-resistant bacteria-induced infection, threatens human health seriously, which has posed great challenges for clinical therapy. The overuse of conventional antibiotics has given rise to bacterial resistance that severely restricts the clinical treatment options of conventional antibiotics. The development of highly effective antibacterial materials and therapeutic strategies to inhibit the multidrug-resistant bacteria-induced infections is of great urgency. Although silver nanoparticles (AgNPs) have exhibited certain effectiveness in killing multidrug-resistant bacteria, their antibacterial efficacy and biosafety are still unsatisfactory. In this work, we prepared graphene quantum dots (GQDs) by a green synthesis method with the natural polymer starch as a precursor for uniformly decorating AgNPs to form GQDs coated AgNPs (GQDs@Ag). The nanocomplex was comprehensively characterized, and its antibacterial activity and biosafety were systematically investigated. The characterization results revealed that the successfully constructed GQDs@Ag hybrids with improved dispersion and stability are composed of AgNPs closely and uniformly surrounded by the GQDs. Furthermore, in vitro and in vivo results demonstrated that GQDs@Ag hybrids with superior biosafety showed a markedly enhanced effect in killing MRSA and accelerating MRSA-infected wound healing as compared to AgNPs alone. Collectively, these results suggest that the biocompatible nanosystem of GQDs@Ag exhibits great potential in clinical application for MRSA infection.

Dual-emission and single-emission carbon dots (DCDs and SCDs) have been simultaneously synthesized by one-pot solvothermal treatment of leek. Different graphitization and surface functionalization were responsible for their distinction in fluorescence characteristics. DCDs with an average size of 5.6 nm exhibited two emissions at 489 and 676 nm under 420-nm excitation. Complexation between DCDs' surface porphyrins and Cu 2+ led to quenching of the 676-nm emission, which resulted in the ratiometric determination of Cu 2+ with a limit of detection (LOD) of 0.085 μM. SCDs, containing additional sulfur element (0.50%) with an average size of 7.7 nm, presented a single emission at 440 nm under 365-nm excitation. The static quenching and inner filter effects between SCDs and tetracyclines (TCs) made SCDs a fluorescence nanoprobe for TCs' determination with LODs of 0.26-0.48 μM. Applications of DCDs and SCDs for respective determination of Cu 2+ and TCs in milk and pig liver samples were successfully demonstrated. Moreover, good photostability, low toxicity, and outstanding biocompatibility made DCDs and SCDs suitable for multicolor cellular imaging. Results indicate that natural products are excellent raw materials to controllably synthesize CDs with prominent physicochemical and fluorescence properties.Graphical abstract.

Nitrite (NO 2 - ) ions are recognized as two typical inorganic contaminations that can cause severe damage to the environment and humans. In this paper, the energy transfer from carbon quantum dots (CQDs) to Tb 2+ ) ions are recognized as two typical inorganic contaminations that can cause severe damage to the environment and humans. In this paper, the energy transfer from carbon quantum dots (CQDs) to Tb 3+ by the antenna effect was found, which can promote the f-f transition of Tb 3+ and emit strong characteristic fluorescence of Tb 3+ . Based on CQD-Tb 3+ and 3-aminophenylboronic acid (APBA), which have respectively sensitive and specific fluorescent response to NO 2 - and Hg 2+ with different fluorescent signals, a dual-emission fluorescent probe of the CQD-Tb 3+ /APBA hybrid was thus fabricated for the simultaneous detection of NO 2 - and Hg 2+ in environmental water samples. They emit dual-emission fluorescence peaks centered at 373 and 545 nm, respectively. A good linearity between the quenching efficiency of ( F 0 - F )/ F 0 and the concentrations of NO 2 - and Hg 2+ was in the range of 5.0-1200.0 nM for NO 2 - and 0.1-6.0 μM for Hg 2+ . The limit of detection (LOD, 3σ/K) is ultrasensitive for NO 2 - (2.0 nM), and the LOD for Hg 2+ is 38.1 nM. The dual-emission fluorescent probe was successfully applied for the determination of NO 2 - and Hg 2+ in various environmental water samples. The possible luminescence and fluorescence quenching mechanisms of the probe are also discussed in detail. This study provides a new approach for fabricating a multifunctional fluorescent probe or sensor and a prospective application in environmental monitoring.

A new-style white pepper derived dual-emission carbon dots (CDs) with a quantum yield of 10.4% was designed and facile constructed with one-pot solvothermal method. The green emission (520 nm) had an efficient and special "turn-on" fluorescence sensing of coenzyme A (CoA) with the aid of Cu 2+ , while red emission (668 nm) barely changed and worked as reference. In the concentration range (0-150 µM), relative fluorescence intensity ratios (F 520 /F 668 ) showed excellent linear correlation with concentrations of CoA, and detection limit was as low as 8.75 nm. Moreover, the strategy has been successfully applied for label-free detection of CoA in real pig liver samples with good recoveries (93.3-108.0%). Notably, the synthesized CDs had durable fluorescence, low cytotoxicity, and good biocompatibility for cellular imaging, which demonstrated wide and promising applicability for biosensing and bioimaging in the future., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Dual-emissive carbon dots (CDs) were fabricated for dual-channel ratiometric fluorometric determination of pH and mercury ion (Hg 2+ ) and intracellular imaging. Dual-emissive CDs were synthesized by one-pot solvothermal treatment of cabbage. The CDs exhibited two distinctive fluorescence emissions at 500 and 678 nm under single excitation at 410 nm. The green emission (500 nm) had reversible linear response to pH (7.0-12.0) due to deprotonation and protonation of surface functional groups and their non-covalent interactions. On the other hand, the red emission (678 nm) had efficient and selective fluorescence response to Hg 2+ by formation of non-emission complex between CDs and Hg 2+ . The limit of detection (LOD) and limit of quantification (LOQ) for Hg 2+ were 6.25 and 20.63 nM, respectively. The CDs have been successfully applied for label-free ratiometric fluorometric determination of pH and Hg 2+ in fish and human serum samples with good recoveries (92.0-108.3%). In addition, the CDs had excellent photostability, low cytotoxicity, and good biocompatibility for intracellular imaging. All in all, the system was multi-functional in determination, high in sensitivity, and excellent in selectivity, which demonstrated wide and promising applicability for biosensing and bioimaging in the future. Graphical abstract Schematic presentation of dual-emission carbon dots (CDs) synthesized by solvothermal treatment of cabbage for dual-channel determination of pH and Hg 2+ .

Aim: Constructing a new drug-delivery system using carboxylated graphene quantum dots (cGQDs) for tumor chemotherapy in vivo . Materials & methods: A drug-delivery system was synthesized through a crosslink reaction of cGQDs, NH 2 -poly(ethylene glycol)-NH 2 and folic acid. Results: A drug delivery system of folic acid-poly(ethylene glycol)-cGQDs was successfully constructed with ideal entrapment efficiency (97.5%) and drug-loading capacity (40.1%). Cell image indicated that the nanosystem entered into human cervical cancer cells mainly through macropinocytosis-dependent pathway. In vivo experiments showed the outstanding antitumor ability and low systemic toxicity of this nanodrug-delivery system. Conclusion: The newly developed drug-delivery system provides an important alternative for tumor therapy without causing systemic adverse effects.

Nitrogen- and sulfur-codoped carbon dots (N,S-CDs) with a fluorescence quantum yield of 16.1% and good photoluminescent properties were synthesized by a simple hydrothermal method. Cytotoxicity of the N,S-CDs was evaluated by the MTT assay, and human hepatoma HepG2 cells were chosen as the target. The cell viability was more than 85% after 24 h of incubation when its concentration was up to 300 μg/mL, suggesting low cytotoxicity and good biocompatibility of the N,S-CDs. The fluorescence spectra of the N,S-CDs are excitation-dependent in the excitation-wavelength range of 295-400 nm, and it emits bright blue fluorescence centered at 435 nm. Its selective fluorescence recognition for Hg 2+ ions was found. When Hg 2+ ions were added to the N,S-CDs solution, its bright blue fluorescence was obviously quenched and could be recovered by the addition of sulfide. Accordingly, a new strategy based on N,S-CDs-Hg 2+  system as a highly selective and ultrasensitive "turn off-on" fluorescence sensing for the detection of sulfide was fabricated. The limits of detection (S/N = 3) are 83 nM for Hg 2+ ions and 11 nM for sulfide. Almost no statistically significant interference for Hg 2+ -ion and sulfide detection was observed among possible coexisting substances in the water samples, including 17 common metal ions and 11 anions. This probe was successfully applied for the cellular imaging of Hg 2+ ions in HepG2 cells by fluorescence microscopy.

Novel thermoresponsive CdTe/ZnS quantum dots (QDs) decorated with a copolymer ligand (CPL) containing 8-hydroxyquinoline and NIPAM units are prepared through coordinate bonding in aqueous solution. The dependence of the morphology and optical properties of the QDs/CPL assemblies formed via coordinate bonding on the experimental conditions is studied. The coordinate induced self-assemblies are observed by controlling the molar ratio of QDs and CPL. The self-organized structure of QDs/CPL proceeds through a first step of QDs-chains, followed by a necklace-like single annular chain, and subsequently increases its annular chain structure, forming a network. The CPL functionalized QDs can emit multiple colors from the cooperating interaction between the inherent emission (606 nm) of the QDs and the surface-coordinated emission (517 nm) of the CPL complex formed on the QD surface. For QDs-CPL systems, both Förster resonance energy transfer (FRET) and a high rate of photoinduced electron transfer (PET) are simultaneous, the latter mainly contributing to PL quenching. The thermoresponsive QDs/CPL assemblies also exhibit dual reversible PL properties between the inherent emission of QDs and surface-coordinated emission., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The self-heating effect in 1.3 μm p-doped InAs/GaAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) has been investigated using a self-consistent theoretical model. Good agreement is obtained between theoretical analysis and experimental results under pulsed operation. The results show that in p-doped QD VCSELs, the output power is significantly influenced by self-heating. About 60% of output power is limited by self-heating in a device with oxide aperture of 5×6 μm2. This value reduces to 55% and 48%, respectively, as the oxide aperture increases to 7×8 and 15×15 μm2. The temperature increase in the active region and injection efficiency of the QDs are calculated and discussed based on the different oxide aperture areas and duty cycle. [ABSTRACT FROM AUTHOR]

The threshold characteristic and output power of 1.3 μm quantum dot (QD) vertical-cavity surface-emitting laser (VCSEL) with dots-under-a-well and dots-in-a-well InAs/GaAs QD structures are investigated by using rate equation model and output power model. The influence of VCSEL and QD structures on the modal gain of VCSEL is analyzed. Threshold current density, quantum efficiency, and characteristic temperature are simulated for different QD structures. The dependence of output power of 1.3 μm QD VCSEL on the QD structure, threshold current, quantum efficiency, and oxide-aperture size is investigated in detail. [ABSTRACT FROM AUTHOR]

GaAs quantum dots formed by Ar+ bombardment under normal beam incidence are investigated in both sputtering time and energy domains. When ion energy is maintained at 1000 eV, the surface morphology is found to saturate with high dot uniformity at 3600 s sputtering time. For longer sputtering times, the surface pattern becomes significantly disordered with fluctuations of ∼28 and ∼24 nm in dot height and base width, respectively. The temporal evolution of dots formed at lower ion energies exhibits a similar trend, as observed at 1000 eV. However, the surface morphology develops in a smaller size scale. Based on the experimental results, we propose a power law model to interpret the correlation between sputtering time and energy as well as their impact on the evolution of lateral dot sizes. The experimental results are in good agreement with the theoretical prediction. Furthermore, photoluminescence is performed to characterize the as-grown and annealed GaAs/AlGaAs quantum dots formed by ion sputtering and molecular beam epitaxy. A significant improvement in the integrated photoluminescence signal has been obtained after thermal annealing, indicating that the potential nonradiative defects can be effectively removed by postgrowth annealing of the sputtering-induced GaAs quantum dot system. [ABSTRACT FROM AUTHOR]

Postgrowth rapid thermal annealing was used to investigate the intermixing and structural changes in p-doped and undoped InAs/In0.1Ga0.9As dots-in-a-well (DWELL) structures grown by molecular beam epitaxy. Interdiffusion of In and Ga atoms caused by thermal annealing was proven from photoluminescence (PL) measurements, where blueshifts of the energy peaks were observed. The results show that p-doped quantum dot (QD) structures are more resistant to intermixing with higher thermal energy onset, and the reason is explained as the suppressed Ga diffusion resulted from the Be dopant. Rapid quenching of the integrated PL intensity at high temperature was observed in both undoped and p-doped DWELL QDs. Good agreement was obtained by fitting the integrated PL profile using two nonradiative recombination mechanisms, resulting in two activation energies that correspond to loss of carriers to nonradiative centers. [ABSTRACT FROM AUTHOR]

The negative characteristic temperature of InAs/GaAs quantum dot lasers is studied using a rate equation model. It is found that the decrease in the total contribution to lasing following a decrease in temperature is the reason for the occurrence of negative characteristic temperature in these lasers. The temperature corresponding to the occurrence of negative characteristic temperature is determined by the carrier escape rate from the quantum dots to the wetting layer or cap layer, carrier recombination lifetime, and rate of carrier loss due to deviation from (quasi-) Fermi equilibrium. The negative characteristic temperature in InAs/GaAs quantum dot lasers does not occur under conditions of low carrier recombination lifetime and high quantum dot energy level occupation. [ABSTRACT FROM AUTHOR]

A self-consistent model comprising rate equations and thermal conduction equation is used to analyze the influence of self-heating on the carrier occupation, quantum efficiency, and output power of 1.3-µm InAs-GaAs quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). The simulation results show that the poor hole confinement in QDs is due to the thin wetting layer, and increase in QD density and layer number can significantly improve the self-heating effect and quantum efficiency of the device. The output power of the QD VCSEL is mainly determined by the quantum efficiency. High output power can be achieved by the high number of QD layers and QD density. However, there exists an optimized number of QD layers (~15) to achieve the highest output power. [ABSTRACT FROM AUTHOR]

A rate-equation model, in which three discrete quantum-dot (QD) energy levels are assumed and all possible relaxation paths and carrier transport in the GaAs barrier are considered, is presented to analyze the steady-state performance of 1.3 µm undoped and doped dots-under-a-well (DUW) and dots-in-a-well (DWELL) InAs-GaAs QD lasers. DWELL QD lasers have higher saturation value of QD level occupation probabilities and characteristic temperature (T0) than that of DUW QD lasers due to the improvement of hole confinement. The p-doped QD laser shows lower threshold current density than n-doped QD laser at the same threshold condition, and the T0 of n-doped DWELL laser is higher than that of p-doped DWELL laser at room temperature. Optimized QD layer number of DUW and DWELL QD lasers with different QD density is discussed.' [ABSTRACT FROM AUTHOR]

The competition of ground state (GS) and excited state (ES) is investigated from the as-grown and thermally annealed 1.3 μm ten-layer p-doped InAs/GaAs quantum dot (QD) lasers. The modal gain competition between GS and ES are measured and analyzed around the ES threshold characteristics. Our results show that two-state competition is more significant in devices with short cavity length operating at high temperature. By comparing the as-grown and annealed devices, we demonstrate enhanced GS and suppressed ES lasing from the QD laser annealed at 600 °C for 15 s. [ABSTRACT FROM AUTHOR]

High temperature photoluminescence up to 100 °C was demonstrated from the p-doped ten-layer InAs/InGaAs quantum dot (QD) laser structure. 1.3 μm InAs QD lasers were fabricated using pulsed anodic oxidation from this structure. High output power of 882 mW and low transparency current density of 5.9 A/cm2/QD layer were obtained. Ground state (GS) lasing could be maintained from a QD laser with short cavity length of 611 μm, corresponding to the maximum modal gain of 23.1 cm-1 from this laser system. GS continuous wave operation up to 100 °C was also demonstrated from an InAs QD laser (50×2500 μm2). [ABSTRACT FROM AUTHOR]

Transport measurements through a few-electron circular quantum dot in bilayer graphene display bunching of the conductance resonances in groups of four, eight, and twelve. This is in accordance with the spin and valley degeneracies in bilayer graphene and an additional threefold "minivalley degeneracy" caused by trigonal warping. For small electron numbers, implying a small dot size and a small displacement field, a two-dimensional s shell and then a p shell are successively filled with four and eight electrons, respectively. For electron numbers larger than 12, as the dot size and the displacement field increase, the single-particle ground state evolves into a threefold degenerate minivalley ground state. A transition between these regimes is observed in our measurements and can be described by band-structure calculations. Measurements in the magnetic field confirm Hund's second rule for spin filling of the quantum dot levels, emphasizing the importance of exchange interaction effects. [ABSTRACT FROM AUTHOR]

We report ground- and excited-state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different from quantum dots in carbon nanotubes, where the two-particle ground state is a spin-singlet state. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35 meV and measure a valley and spin g factor of 36 and 2, respectively. [ABSTRACT FROM AUTHOR]