Carbon quantum dots (CQDs) derived from biomass, a suggested green approach for nanomaterial synthesis, often possess poor optical properties and have low photoluminescence quantum yield (PLQY). This study employed an environmentally friendly, cost-effective, continuous hydrothermal flow synthesis (CHFS) process to synthesise efficient nitrogen-doped carbon quantum dots (N-CQDs) from biomass precursors (glucose in the presence of ammonia). The concentrations of ammonia, as nitrogen dopant precursor, were varied to optimise the optical properties of CQDs. Optimised N-CQDs showed significant enhancement in fluorescence emission properties with a PLQY of 9.6% compared to pure glucose derived-CQDs (g-CQDs) without nitrogen doping which have PLQY of less than 1%. With stability over a pH range of pH 2 to pH 11, the N-CQDs showed excellent sensitivity as a nano-sensor for the highly toxic highly-pollutant chromium (VI), where efficient photoluminescence (PL) quenching was observed. The optimised nitrogen-doping process demonstrated effective and efficient tuning of the overall electronic structure of the N-CQDs resulting in enhanced optical properties and performance as a nano-sensor.
Introduction
The demand for high-performance carbon quantum dots (CQDs) with a range of applications, including sensing has been steadily increasing. However, the synthesis of CQDs continues to face challenges including high costs, lengthy multistep processes, and the use of hazardous substances1,2. Recently, biomass-derived CQDs have attracted considerable attention, and are considered as an optimal and green approach to prepare efficient CQDs. Biomass and biomass waste (agriculture product, agricultural residue, municipal solid waste etc.) are abundant, high in carbon content (45–55%), and are an environmentally friendly renewable resource3. Therefore, the utilisation of biomass as carbon resources for nanomaterial synthesis is an eco-friendly process and expected to reduce the total synthetic cost4. Although a broad range of biomass materials have been employed in producing CQDs, generally, these synthetic routes faced problems associated with poor control of the CQDs particle size, quality, and homogeneity of the product5. In addition, the CQDs synthesised from biomass or biomass waste, commonly possess poor optical properties and a low PLQY. The doping of heteroatom such as (N, P, S) is one of the most common methods to improve the optical properties of biomass-derived CQDs6,7. However, the questions related to the origin of the optical improvement with optimised dosing of these heteroatoms still need to be answered. Furthermore, in most conventional methodologies, these doping processes result in a longer synthesis time and higher energy consumption8.
In this work, the continuous hydrothermal flow synthesis (CHFS) which is primarily water-based was employed; thus, it is considered the greenest and most promising synthesis method for making CQDs. Notably, the CHFS allows designing or tailoring of the nanoparticles for specific functions based on the nucleation and surface functional processes. The comparison between CHFS and the traditional hydrothermal process revealed that the CHFS consumed less energy and time, while producing a highly homogenous quality product9. Moreover, the continuous hydrothermal process can be employed in multi-purposes such as controlling the nucleation to control the particle size and the addition of surfactant coating or dopant without further post-treatments10. In this paper, we report the use of CHFS process, to successfully synthesise N-doped carbon quantum dots (N-CQDs) from glucose which is an abundant, readily available, cost-effective biomass carbon source; and ammonia is used as a nitrogen dopant. Synthesised N-CQDs with different concentrations of ammonia were used to explore the effect of the concentration of nitrogen dopant on the optical characteristics of CQDs. A range of characterisation techniques were employed to investigate the origin of the optical enhancement. The performance of the N-CQDs as chemical nanosensor was tested. Currently clean water resources are foremost among global challenges facing society today. A significant proportion of the worlds wastewater containing heavy metals as pollutants is disposed untreated in to the environment11. Therefore, the application of the prepared N-CQDs as chemical sensor to detect chromium (VI) which is carcinogenic, hemotoxic, and genotoxic; the main source being industrial waste water, would be timely.
Experimental work
Chemicals
Glucose, ammonia (32%), potassium chromate, and potassium dichromate were purchased from Fisher Scientific. The solutions of metal ions used for the sensing application experiments were prepared using nitrate (Ag+, Ce3+, Co2+, Cr3+, Ni2+, Fe3+), sulphate (Cu2+ and Fe2+), chloride (K+, Na+, Mg2+) and sodium (CrO42−, Cr2O72−, NO3−, CH3COO−, HCOO−, SO42−, F−, Cl−, Br−, I−). These chemicals were purchased from Sigma-Aldrich and were used as received. 15 MΩ deionized H2O (ELGA Purelab) was used for all the experiments.
Equipment
UV–Vis spectrophotometry: Shimadzu UV-1800 was used to perform the absorption measurements (λ = 200 to 800 nm) using a quartz cuvette (10 mm).
Photoluminescence (PL) spectroscopy: The steady-state fluorescence spectra of NCQDs were measured with Shimadzu RF-6000 spectrofluorophotometer.
High-resolution transmission electron microscopy (HRTEM): NCQDs were diluted in isopropanol and applied onto a carbon holey mesh grid (Agar) and allowed to air dry. The samples were then imaged using JEM 2100 (Jeol, Japan) at an acceleration voltage of 200 kV and at a range of magnifications between 15 and 500 K. Representatives NCQDs samples (g-CQDs, N-0.25, N-1, N-5 and N-10) were imaged and analysed.
Fourier–transform infrared (FTIR) spectra was recorded using an IR Affinity-1S Fourier transform infrared spectrometer instrument.
Raman spectra of the prepared N-CQDs was measured with a Horiba LabRAM HR Evolution spectrometer with radiation at 633 nm.
An Edinburgh Instruments FLS1000 photoluminescence spectrometer was used to measure the PL lifetime and the PLQY of the samples. The lifetime was measured using a 375 nm pulsed laser, and the data was fitted with 3-exponentials after reconvolution with instrument response function. The absolute quantum yield (QYabs) of the samples were investigated by using integrating sphere accessory with a standard method. Then, the true fluorescence quantum yield (QYtrue) is calculated by using the Eq. (1) where a is the fraction of the re-absorbed area.
X-ray photoelectron spectroscopy (XPS): an AXIS Ultra DLD (Kratos Surface Analysis) setup equipped with a 180° hemispherical analyser, using Al Kα1 (1486.74 eV) radiation produced by a mono-chromicized X-ray source at operating power of 300 W (15 kV × 20 mA), with spot size of 0.7 mm was used to record the XPS spectra. The partial charge compensation was achieved by using a flood gun operating at 1.52 A filament current, 2.73 V charge balance, and 1.02 V filament bias. The vacuum in the analysis chamber was at least 1 × 10−8 mbar.
Synthetic methodology
Continuous hydrothermal flow synthesis (CHFS) was employed to synthesize N-CQDs (Supplementary Fig. 1). The process consists of three feedstock streams; (i) glucose (with a concentration of 70 mg mL−1) which was used as carbon source, (ii) ammonia with varied concentrations from 0.25 M up to 10.0 M was used as an N-dopant, and (iii) supercritical water which is the key parameter of this reaction. Firstly, the deionized water (with the flow rate 20 mL min−1) was heated up to 450 °C, and the pressure was kept at 24.8 MPa by using a back-pressure regulator (BPR) during the experiment (this condition is previously reported by our group as the optimised environment for the CQDs synthesis)10. The reaction was conducted by injecting the two precursors into the engineered mixer labelled as the “Reactor” (Fig. S1). Here, the precursors were mixed with supercritical water, and the nano dots were produced (in fraction of seconds). The residence time (~ 1.8 s) of the reaction was controlled by the flow rate of the precursors; both glucose and ammonia were pumped at the same time into the reactor with 5 mL min−1 flow rate. The reaction mixture travelled through a cooler to the BPR, and was collected for further treatments. The obtained solutions from the CHFS reaction mixtures were filtered using a 0.2 µm alumina membrane; subsequently, the solutions were continuously dialysed using a 30 kDa membrane in a tangential filtration unit. The cleaned solutions were freeze-dried, and the obtained average yield was 10.68 mg ml−1.
The CQDs samples synthesised from [glucose] = 70 mg mL−1 and ammonia with varied concentrations (0.25 M, 0.5 M, 0. 75 M, 1.0 M, 2.5 M, 5.0 M, 7.5 M and 10.0 M), were denoted as N-0.25, N-0.5, N-0.75, N-1, N-2.5, N-5, N-7.5, and N-10, respectively; g-CQDs was synthesised from the same source (glucose) but without nitrogen doping. The fluorescent photographs of the samples are shown in Fig. S2.
pH stability testing
The solutions with pH ranging from 1 to 13 were prepared using varying concentrations of NaOH (initial concentration 1.0 M) and HCl (1.0 M) solutions which were then diluted to the required pH. A dilute solution of N-CQDs (optical density, OD = 0.1) in deionised water was prepared. Following that, 100 μL of this diluted N-CQDs solution was added to a 3000 μL of each solution prepared at thedesired pH level (range 1–13). A pH meter was used to measure the corresponding pH values. The fluorescence spectra of these solutions were recorded using a Shimadzu RF-6000 Spectro fluorophotometer.
Chromium (VI) ion-sensing experiments
The detection of Cr (VI) ion experiment was conducted with various metal ions (as reported above), each prepared with a concentration of 50 ppm. In a typical experiment, 100 μL, N-CQDs (0.1 OD) was added to the 3.0 mL aqueous metal ion solution. The fluorescence spectrum of mixture solutions was measured using a Shimadzu RF-6000 Spectro fluorophotometermeas. The fluorescence lifetime was investigated to achieve a deeper understanding of the quenching mechanism using an Edinburgh Instruments FLS1000 spectrophotometer.
Limits of detection (LOD) and limits of quantification (LOQ)
The sensitivity of the N-CQDs sensor for Cr (VI) was investigated by evaluating their LOD and LOQ. For that, Cr (VI) ion solutions with various concentrations of 300 ppm, 200 ppm, 100 ppm, 50 ppm, 30 ppm, 10 ppm, 5 ppm, 2 ppm, 1 ppm and 0.5 ppm were first prepared. Then, 100 μL of N-CQDs were added to 3.0 mL of the prepared Cr (VI) ion solutions. The fluorescence spectra were recorded to estimate the LOD and LOQ by using Stem-Volmer graphs, LOD = 3σ/Ksv, LOQ = 10σ/Ksv, where Ksv is the slope of the graph, and σ is the error of the intercept.
Results and discussion
HRTEM images of N-CQDs (representative N-0.25 sample) show that the as-synthesized N-CQDs are spherical (Fig. 1a,c) with particle size ranging from 1.78 to 6.50 nm. The Gaussian distribution (Fig. 1b) of a sample of 150 particles shows the mean particle size of 4.60 ± 0.87 nm. In addition, the N-CQDs possess a crystalline structure as indicated by graphite lattice d-spacing of 0.22 nm (Fig. 1d). Similar features were also observed for the other samples g-CQDs, N-1, N-5 and N-10 analysed via TEM (Fig. S3).
Figure 1
HRTEM images of N-CQDs at different magnification and scale: (a) 20 nm, (c) 10 nm, (b) particle size Gaussian distribution histogram, (d) graphitic core lattice. The N-CQDs have commonly a particle size of 4.60 ± 0.87 nm.
To determine the nature of the functionalisation, the synthesised N-CQDs were investigated using the Fourier transform infrared (FTIR) spectroscopy. The samples were classified into two groups:(i) N-CQDs with a lower concentration of ammonia (from N-0.25 to N-1) and (ii) with a higher concentration of ammonia (from N-2.5 to N-10). The FTIR spectra (Fig. 2) showed that all N-CQDs have hydrophilic groups on their surface such as O–H (hydroxyl) corresponding to the peak at 3389 cm−1 and N–H (3263 cm−1), thus confirming their good solubility in the water. In addition, vibrations of C–H (2950 cm−1), C=O (1581 cm−1), C–N (1435 cm−1) and C–O (1080 cm−1) bonds were also observed in each sample13,14,15. The comparison of the FTIR spectra (Fig. S4) of the samples showed that increasing N-doping (ammonia concentration, from N-0.25 to N-1) displayed a diminishing stretch in vibration for C–O bond at 1080 cm−1. While the group of samples with a higher concentration of ammonia (N-2.5 to N-10) showed a sharp vibration of C–N bond at 1435 cm−1.
Figure 2
FTIR spectra of N-CQDs with a lower concentration of ammonia (from N-0.25) and higher concentration of ammonia (N-10).
To achieve a deeper understanding of the surface characterisation of the N-CQDs and also to investigate the chemical composition of N-CQDs, X-ray photoelectron spectroscopy (XPS) was employed. The resultant XPS spectra shown in Fig. 3 were deconvoluted using Voigt functions (Lorentzian and Gaussian widths) with a distinct inelastic background for each component16. A minimum number of components is used to obtain a convenient fit. The binding energy scale was calibrated to the C 1s standard value of 284.6 eV. The atomic composition has been determined by using the integral areas provided by the deconvolution procedure normalized at the atomic sensitivity factor (Table S1). The XPS spectrum of the N-CQDs displays three typical peaks C1s (285.0 eV), N1s (399.0 eV), and O1s (531.0 eV). The fitted C1s spectrum was deconvoluted into four components, corresponding to carbon in form of C=C/C–C bonds (~ 284.4 eV), C–O/C–N (~ 285.8 eV), C=O (~ 287.3) and O=C–OH (~ 288.4 eV)17. Whilst, the N1s band showed three peaks after deconvolution which are 398.8 eV, 399.6 eV and 400.8 eV, representing pyridinic N, N–H and amide C–N, respectively18.
Figure 3
Representative XPS spectra of N-CQDs showing the lowest (N-0.25) and highest (N-10) nitrogen doped samples. The spectra display three typical peaks C1s (285.0 eV), N1s (399.0 eV), and O1s (531.0 eV). The deconvoluted N1s band showed three peaks representing pyridinic N, N–H and amide C–N.
The content of each nitrogen doping species (pyridinic, pyrrolic and graphitic) are identified and quantified from the XPS spectra of NCQDs with the purpose of understanding their influence over the optical and chemical properties (Table S2). As commonly reported, the fluorescent property of CQDs can be enhanced by using nitrogen-doping. However, only the nitrogen bonded to carbon can improve the emission19. Also, a larger ratio of N/C was observed for N-CQDs samples synthesised with a higher concentration of ammonia (Table S1). The O1s region contains three peaks at 530.9 eV, 532.2 eV and 533.3 eV for C–OH/C–O–C, C=O, H–O–H, respectively20. In addition, the oxygen content is also a key parameter in the N-CQDs emission as it can maintain the balance between sp2 and sp3-carbon atoms21. Therefore, Raman spectroscopy was employed to investigate the disorder in the carbon bonding arrangement of N-CQDs.
The Raman spectra (Fig. S5) of the N-CQDs exhibited typical graphitic features consisting of the D mode (at 1368 cm−1) related to symmetry transformation by the defects, and the G band (at 1586 cm−1), which is assigned to the graphitic core sp2 (graphite-like) bonds. This is not surprising as the HRTEM images of N-CQDs showed a typical lattice spacing of graphite (see Fig. 1d). When comparing the Raman spectra between N-CQDs, at first glance, those spectra look similar, a common ratio ID/IG of 0.95 revealed a balance between the sp2 and sp3 bonds in the N-CQDs structure. This is different from g-CQDs where an ID/IG ratio of 0.83 was observed and assigned to the carbon core (sp2 bonds)10. This is probably attributed to the changes introduced by the nitrogen doping resulting in the transformation of C–C (sp2 bonds) into the sp3 bonding between N, O and C.
Optical properties of N-CQDs
The absorption spectra of the as-prepared N-CQDs measured using UV–Vis spectrophotometry are shown in Fig. 4. The N-CQDs samples have a strong peak around 265 nm and a shoulder around 295 nm (Fig. 4a). The 265 nm absorption peak is characteristics of π–π* transitions of the graphitic core (C=C or C–C) of sp2 domains present in the sp3 environment, and the 295 nm is attributed to n–π* (C=O) transitions and C–N/C=N bonds22,23. For comparison, the absorption spectrum of CQDs without nitrogen doping was also measured. It is noted that the absorption peaks related to N-CQDs are red-shifted compared to g-CQDs (synthesised from the same source glucose but without nitrogen doping), Fig. 4b. These transitions, are observed at 225 nm = π–π* (graphitic core), and 280 nm = n–π* transitions (C=O)10. Therefore, the absorption peak observed at 295 nm in the case of the N-CQDs is due to the formation of the C–N/C=N bonds related to the doping effect caused by the presence of graphitic nitrogen24,25.
Figure 4
(a) UV–Vis absorption spectra of (a) N-CQDs and (b) g-CQDs without nitrogen doping. The presence of C–N/C=N bonds is observed at 295 nm.
The photoluminescence (PL) spectra of as-prepared N-CQDs were measured using a range of different excitation wavelengths as shown in Fig. 5. The PL emission of each sample clearly showed the excitation-dependent PL which is beneficial for a variety of applications such as biosensors, bio-images, or LED devices26,27. The PL emission peaks shifted when different excitation wavelengths were applied, and each sample exhibited an optimal excitation wavelength. Overall, the PL study revealed interesting optical properties of the N-CQDs. Firstly, the PL results are consistent with previous reports where the excitation dependent emission phenomenon of CQDs was observed28. Secondly, the maximum excitation wavelengths varied from 360 to 320 nm with the concentration of ammonia.
Figure 5
Photoluminescence spectra of CQD with and without nitrogen doping measured using excitation wavelengths in the range of 300 to 500 nm, (a) g-CQDs (without nitrogen doping); (b) N-0.25; (c) N-2.5, (d) N-10.
However, the mechanisms behind the excitation dependent properties of CQDs is not clear. One of the most comprehensive and broadly accepted mechanism in interpreting the excitation-dependent PL of the CQDs is the quantum confinement effect also known as the size effect14,21,28,29. In general, the CQDs possess broad particle size distributions which leads to a range of different energy gaps and is the reason for the variation of emission wavelengths30,31. But herein, HRTEM image data analyses confirmed that the increased amount of nitrogen doping did not contribute to an increase in the particle size for the as prepared samples. Therefore, the observed red-shift character can be ascribed to the radiative recombination of e−h pairs hosted in the sp2 clusters32. Aside from the quantum confinement effect, surface states theory is rather broadly adopted to interpret the excitation-dependent PL behaviour of CQDs33,34,35. UV–Vis absorbance showed that the peak of the N-CQDs at 265 nm is related to the π–π* transition, which suggests the existence of a large number of π-electrons. The surface electronic states can conjugate with these π-electrons as a results of the surface oxidation which result in the modification of the electronic structure of the N-CQDs34,36.
To interpret the mechanism of this effect, the PL lifetime and PLQY of N-CQDs were measured. The obtained results (Table 1) showed an increase in both PL lifetime and PLQY upon nitrogen doping and the highest values of lifetime and PLQY were obtained for (left[Nright]ge 7.5 M). The obtained PLQY value of 9.6%(pm) 0.9 for N-10 is an significant improvement compared to g-CQDs which showed PLQY of < 1%10. These results are comparable to the literature (shown in Table S3), where CQDs and N-CQDs were synthesised via different methodologies.
Table 1 The photoluminescence quantum yield (PLQY), average lifetime, 1/e lifetime, radiative (kr) and non-radiative (knr) rates of N-CQDs.
The radiative rate (kr) and non-radiative rate (knr) were calculated by using the Eq. (2) and (3) 37.
$$k_r=fracPhi tau _1/e$$
(2)
$$Phi =frack_rk_r+k_nr$$
(3)
where (Phi) is PLQY of N-CQD and (tau _1/e) corresponds to the lifetime when fluorescence drops 1/e of its initial value.
Table 1 and Fig. 6 shows that when a higher concentration of ammonia was used, the non-radiative rates significantly reduced. This is due to surface coating activities of the nitrogen functional group which led to enhanced PLQY38. In addition, the lower non-radiative constant suggested that N-CQDs possess an efficient recombination process which led to an observation of nanosecond scale PL lifetime. These recombination processes suggested strong coupling of excited core states with the surface state. Thus confirming that the π-electron systems affect the surface electronic state leading to the modification of the overall electronic structure of N-CQDs36,39.
Figure 6
(a) PLQY and non-radiative rate (GlU = g-CQDs), (b) PL lifetime of g-CQDs and N-CQDs. The analysis revealed an increase in both PL lifetime and PLQY upon nitrogen doping and the highest values of lifetime and PLQY were obtained for [N] ≥ 7.5 M.
The stability of CQDs for a broad pH range is essential for sensing applications. Therefore, PL of the N-CQDs in pH solution was measured to establish the relationship between the pH level and the emission intensity. N-CQDs showed fluorescence stability in a broad pH range from 2 to 11. For example, the fluorescence intensity of sample N-0.25 (Fig. 7a,b) was dramatically reduced by ~ 60% at pH 1, ~ 35% at pH 12 and ~ 40% at pH 13; and there were slight decreases at pH 11 (~ 12%).
Figure 7
pH effect on the emission intensity of N-CQDs. Representative samples selected to show the highest and lowest [N] doping levels: (a,b) N-0.25, (c,d) N-10.
The diminishing fluorescent intensity behaviour of N-CQDs in strongly acidic and alkali media, noticeable also for samples with high content of nitrogen (for example, sample N-10, Fig. 7c,d). This can be related to protonation/deprotonation of the surface functional groups which causes surface charge disruption13. Whilst H-bonding is eliminated by deprotonation in basic conditions which can cause irregular energy levels resulting in the reduction of the N-CQDs fluorescence40. In addition, H+ can introduce surface defects on CQDs by breaking the passivated OH– shell resulting in PL decreasing and a redshifted spectrum41. Indeed, as shown in Fig. 7c, a 20 nm red shift was also noted for the N-CQD in the strong acidic condition (pH 1). We have previously assigned this to the prominent emissions deriving from the graphitic core10.
Chromium (VI) ion-sensing
The N-CQDs were investigated for ion-sensing applications for a series of cations and anions (Fig. 8 and Fig. S6) including chromium (VI) ion (CrO42−/Cr2O72−) which is a major anthropogenic pollutant in industrial wastewater and soils11. The obtained results indicated that N-CQDs are highly sensitive and showed high selectivity towards hexavalent chromium in comparison to a series of other cations and anions.
To identify the sensitivity, limits of detection (LOD) and limits of quantification (LOQ) of N-CQDs were determined by measuring the fluorescent emission quenching as a function of Cr (VI) concentration (Fig. 9a). The reduction in emission intensity with the increase in concentrations of chromium was observed (Fig. 9b) i.e. there is a correlation between them. This correlation was fitted with a linear equation (y=mx+c), where slope m gives the value of quenching constant Ksv and c is the intercept. The LOD and LOQ were determined using the following equations: LOD = 3σ/Ksv, LOQ = 10σ/Ks, respectively, where σ is the standard error of the intercept. The plotted Stern–Volmer graph for N-10 in Fig. 9b was fitted with y = 0.0238x + 0.026 (R2 = 0.9999) which gave a quenching constant value of Ksv = 0. 0238 and intercept of 0.026. The LOD of 0.955 ppm and LOQ of 3.182 ppm were obtained with a standard error of the intercept of 0.0076. The calculated LOD and LOQ for all the samples are shown in Fig. 10. Our results showed a significant improvement compared to previously reported research10 where LOD of 3.62 ppm and LOQ of 11.6 ppm) were reported for g-CQDs. In addition, the obtained results are also comparable to other reported literature (Table S4).
Figure 9
The effect of concentration in the PL intensity of N-10: (a) Stem–Volmer graphs as a function of (log(F0/F) versus Cr (VI) concentration (b).
(a) Spectral overlap of the normalized UV–Vis absorption bands for the Cr (VI) ions (dash black) and the synthesised N-CQDs (green line), and the excitation spectrum (λem = 420 nm) (red line) and emission spectrum (λext = 340 nm) (blue line) of the N-CQDs. (b) LOD and LOQ of N-CQDs.
To understand the quenching mechanism, the change in PL lifetime of N-CQDs in various ion solutions were studied. The mechanism for this fluorescence quenching behaviour in the presence of Cr (VI) can be assigned to Inner Filter Effect (IFE) which is a physical phenomenon that occurs in a sensing system when the absorption spectrum of the absorber has an overlap with that of excitation and/or emission of the fluorescence leading to the reduced fluorescent emission intensity13. The IFE quenching is not related to the radiative and non-radiative transitions in the CQD, thus the intrinsic fluorescence emission is not changed in the presence of the quencher molecule42.
As illustrated in Fig. 10a, the N-CQDs excitation and emission bands (λex = 340 nm and λem = 420 nm) overlapped with the chromate (CrO42−) anions absorption bands at 372 nm. Moreover, CrO42− shows a second absorption band at 274 nm that also overlaps with the N-CQDs most intense absorption band at 265 nm. Further, the fluorescence lifetime of N-CQDs did not change with the addition of Cr (VI) (Fig. S7 and Table S5) which provides an evidence that the IFE is the mechanism for the fluorescence quenching phenomenon. There is a downtrend of N-CQDs in the LOD and LOQ results (Fig. 10b). The LOD decreased from 4.9 ppm for sample N-0.25 to 0.95 ppm for sample N-10, while the LOQ improved from 16.3 ppm to 3.18 ppm. These results revealed that nitrogen doping enhanced the fluorescent properties of CQDs resulting in higher PLQY which then leads to the improvement in LOD and LOQ and consequently the sensing performances.
Conclusions
In conclusion, efficient N-CDQs were synthesised using biomass precursors (glucose) and ammonia via the CHFS process. The synthesized N-CQDs possess excellent optical properties with a PLQY of ~ 10% and showed excellent pH stability (for pH 2 to 11). The synthesized N-CQDs were tested as a chemical sensor for Cr (VI) ion and the LOD value of 0.95 ppm and LOQ value of 3.18 nm were obtained. The fluorescence lifetime studies confirmed Inner Filter Effect (IFE) as the mechanism for the quenching behaviour of the nano-sensing. Hence, this work presented a novel, rapid, single-step and green approach for nanomaterials synthesis in general and carbon quantum dots in particular which then can be used for a range of different applications.
Data availability
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. For the purpose of open access, the authors have applied a Creative Commons attribution (CC BY) licence to any author accepted manuscript version arising.
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SK, MTS, KGN and IAB would like to acknowledge LSBU for all the financial support provided in the completion of this research work. IAB and AN would like to acknowledge funding through the Core Program 21N/2019 and the Project ELI_17/2020 (granted by the Institute of Atomic Physics) of the National Institute of Materials Physics.
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Authors and Affiliations
London Centre for Energy Engineering (LCEE), School of Engineering, London South Bank University, 103 Borough Road, London, SE1 0AA, UK
K.N.G. carried out the experimental work; I.B.: Raman analysis; A.N.: XPS analysis; N.P.P.: facilitated access to TEM facilities, R.G.: TEM characterisation; S.T., A.R. and M.T.S. supported with TRPL analysis; S.K.: principal investigator; K.N.G, M.T.S. and S.K. wrote the paper. All authors reviewed the manuscript.
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Nguyen, K.G., Baragau, IA., Gromicova, R. et al. Investigating the effect of N-doping on carbon quantum dots structure, optical properties and metal ion screening. Sci Rep12, 13806 (2022). https://doi.org/10.1038/s41598-022-16893-x
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More than 40 trillion gallons of rain drenched the Southeast United States in the last week from Hurricane Helene and a run-of-the-mill rainstorm that sloshed in ahead of it — an unheard of amount of water that has stunned experts.
That’s enough to fill the Dallas Cowboys’ stadium 51,000 times, or Lake Tahoe just once. If it was concentrated just on the state of North Carolina that much water would be 3.5 feet deep (more than 1 meter). It’s enough to fill more than 60 million Olympic-size swimming pools.
“That’s an astronomical amount of precipitation,” said Ed Clark, head of the National Oceanic and Atmospheric Administration’s National Water Center in Tuscaloosa, Alabama. “I have not seen something in my 25 years of working at the weather service that is this geographically large of an extent and the sheer volume of water that fell from the sky.”
The flood damage from the rain is apocalyptic, meteorologists said. More than 100 people are dead, according to officials.
Private meteorologist Ryan Maue, a former NOAA chief scientist, calculated the amount of rain, using precipitation measurements made in 2.5-mile-by-2.5 mile grids as measured by satellites and ground observations. He came up with 40 trillion gallons through Sunday for the eastern United States, with 20 trillion gallons of that hitting just Georgia, Tennessee, the Carolinas and Florida from Hurricane Helene.
Clark did the calculations independently and said the 40 trillion gallon figure (151 trillion liters) is about right and, if anything, conservative. Maue said maybe 1 to 2 trillion more gallons of rain had fallen, much if it in Virginia, since his calculations.
Clark, who spends much of his work on issues of shrinking western water supplies, said to put the amount of rain in perspective, it’s more than twice the combined amount of water stored by two key Colorado River basin reservoirs: Lake Powell and Lake Mead.
Several meteorologists said this was a combination of two, maybe three storm systems. Before Helene struck, rain had fallen heavily for days because a low pressure system had “cut off” from the jet stream — which moves weather systems along west to east — and stalled over the Southeast. That funneled plenty of warm water from the Gulf of Mexico. And a storm that fell just short of named status parked along North Carolina’s Atlantic coast, dumping as much as 20 inches of rain, said North Carolina state climatologist Kathie Dello.
Then add Helene, one of the largest storms in the last couple decades and one that held plenty of rain because it was young and moved fast before it hit the Appalachians, said University of Albany hurricane expert Kristen Corbosiero.
“It was not just a perfect storm, but it was a combination of multiple storms that that led to the enormous amount of rain,” Maue said. “That collected at high elevation, we’re talking 3,000 to 6000 feet. And when you drop trillions of gallons on a mountain, that has to go down.”
The fact that these storms hit the mountains made everything worse, and not just because of runoff. The interaction between the mountains and the storm systems wrings more moisture out of the air, Clark, Maue and Corbosiero said.
North Carolina weather officials said their top measurement total was 31.33 inches in the tiny town of Busick. Mount Mitchell also got more than 2 feet of rainfall.
Before 2017’s Hurricane Harvey, “I said to our colleagues, you know, I never thought in my career that we would measure rainfall in feet,” Clark said. “And after Harvey, Florence, the more isolated events in eastern Kentucky, portions of South Dakota. We’re seeing events year in and year out where we are measuring rainfall in feet.”
Storms are getting wetter as the climate change s, said Corbosiero and Dello. A basic law of physics says the air holds nearly 4% more moisture for every degree Fahrenheit warmer (7% for every degree Celsius) and the world has warmed more than 2 degrees (1.2 degrees Celsius) since pre-industrial times.
Corbosiero said meteorologists are vigorously debating how much of Helene is due to worsening climate change and how much is random.
For Dello, the “fingerprints of climate change” were clear.
“We’ve seen tropical storm impacts in western North Carolina. But these storms are wetter and these storms are warmer. And there would have been a time when a tropical storm would have been heading toward North Carolina and would have caused some rain and some damage, but not apocalyptic destruction. ”
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It’s a dinosaur that roamed Alberta’s badlands more than 70 million years ago, sporting a big, bumpy, bony head the size of a baby elephant.
On Wednesday, paleontologists near Grande Prairie pulled its 272-kilogram skull from the ground.
They call it “Big Sam.”
The adult Pachyrhinosaurus is the second plant-eating dinosaur to be unearthed from a dense bonebed belonging to a herd that died together on the edge of a valley that now sits 450 kilometres northwest of Edmonton.
It didn’t die alone.
“We have hundreds of juvenile bones in the bonebed, so we know that there are many babies and some adults among all of the big adults,” Emily Bamforth, a paleontologist with the nearby Philip J. Currie Dinosaur Museum, said in an interview on the way to the dig site.
She described the horned Pachyrhinosaurus as “the smaller, older cousin of the triceratops.”
“This species of dinosaur is endemic to the Grand Prairie area, so it’s found here and nowhere else in the world. They are … kind of about the size of an Indian elephant and a rhino,” she added.
The head alone, she said, is about the size of a baby elephant.
The discovery was a long time coming.
The bonebed was first discovered by a high school teacher out for a walk about 50 years ago. It took the teacher a decade to get anyone from southern Alberta to come to take a look.
“At the time, sort of in the ’70s and ’80s, paleontology in northern Alberta was virtually unknown,” said Bamforth.
When paleontogists eventually got to the site, Bamforth said, they learned “it’s actually one of the densest dinosaur bonebeds in North America.”
“It contains about 100 to 300 bones per square metre,” she said.
Paleontologists have been at the site sporadically ever since, combing through bones belonging to turtles, dinosaurs and lizards. Sixteen years ago, they discovered a large skull of an approximately 30-year-old Pachyrhinosaurus, which is now at the museum.
About a year ago, they found the second adult: Big Sam.
Bamforth said both dinosaurs are believed to have been the elders in the herd.
“Their distinguishing feature is that, instead of having a horn on their nose like a triceratops, they had this big, bony bump called a boss. And they have big, bony bumps over their eyes as well,” she said.
“It makes them look a little strange. It’s the one dinosaur that if you find it, it’s the only possible thing it can be.”
The genders of the two adults are unknown.
Bamforth said the extraction was difficult because Big Sam was intertwined in a cluster of about 300 other bones.
The skull was found upside down, “as if the animal was lying on its back,” but was well preserved, she said.
She said the excavation process involved putting plaster on the skull and wooden planks around if for stability. From there, it was lifted out — very carefully — with a crane, and was to be shipped on a trolley to the museum for study.
“I have extracted skulls in the past. This is probably the biggest one I’ve ever done though,” said Bamforth.
“It’s pretty exciting.”
This report by The Canadian Press was first published Sept. 25, 2024.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
