Synthesis of green-emitting carbon quantum dots with double carbon sources and their application as a fluorescent probe for selective detection of Cu2+ ions

Green-emitting carbon quantum dots (G-CQDs) were prepared using tartaric acid and bran by one-pot solvothermal treatment and had photoluminescence quantum yields (PL QY) as high as 46%. The morphology of the G-CQDs is characterized by TEM, which shows the average diameter of G-CQDs is approximately ∼4.85 nm. The FT-IR spectra display the presence of –OH, C–N, N–H and –COOH on the surface of the G-CQDs. The emission wavelength of the G-CQDs was ∼539 nm in the case of ∼450 nm excitation wavelength, which corresponds to the green fluorescence. Furthermore, the G-CQDs were used as a fluorescent probe for detection Cu²⁺ ions, and demonstrated a linear distribution between ln(F/F₀) and the Cu²⁺ ions concentration. Specifically, the Cu²⁺ ion concentration should fall in the G-CQD concentration range of 0–0.5 mM and the detection limit is 0.0507 μM. Thus, due to the excellent chemical stability and good luminescence performance, these G-CQDs could be excellent probes widely used in detection fields.

Green-emitting carbon quantum dots (G-CQDs) were prepared using tartaric acid and bran by one-pot solvothermal treatment and had photoluminescence quantum yields (PL QY) as high as 46%.     

Tagetes erecta as an organic precursor: synthesis of highly fluorescent CQDs for the micromolar tracing of ferric ions in human blood serum

The present article illustrates the green synthesis of novel carbon quantum dots (CQDs) from biomass viz. Tagetes erecta (TE), and subsequently fabrication of a metal ion probe for the sensing of Fe³⁺ in real samples. TE-derived CQDs (TE-CQDs) have been synthesized by a facile, eco-friendly, bottom-up hydrothermal approach using TE as a carbon source. The successful synthesis and proper phase formation of the envisaged material has been confirmed by various characterization techniques (Raman, XRD, XPS, TEM, and EDS). Notably, the green synthesized TE-CQDs show biocompatibility, good solubility in aqueous media, and non-toxicity. The as-synthesized TE-CQDs show an intense photoluminescence peak at 425 nm and exhibit excitation dependent photoluminescence behavior. The proposed TE-CQD-based probe offers a remarkable fluorescence (FL) quenching for Fe³⁺ with high selectivity (K_q ∼ 10.022 × 10¹³ M⁻¹ s⁻¹) and a sensitive/rapid response in a linear concentration range 0–90 μM (regression coefficient R² ∼ 0.99) for the detection of Fe³⁺. The limit of detection (LOD) of the probe for Fe³⁺ has been found as 0.37 μM in the standard solution. It has further been applied for the detection of Fe³⁺ in real samples (human blood serum) and displays good performance with LOD ∼ 0.36 μM. The proposed TE-CQD-based ion sensing probe has potential prospects to be used effectively in biological studies and clinical diagnosis.

TE-CQDs synthesized via the hydrothermal method for the detection of Fe³⁺ in HBS.     

Differential pulse voltammetry detection of Pb(ii) using nitrogen-doped activated nanoporous carbon from almond shells

Almond shell-based charcoal was prepared by carbonizing almond shells in a nitrogen atmosphere. Nanoporous carbon (NPC) was formed via activating the obtained charcoal using potassium hydroxide as an activating agent, followed by the synthesis of nitrogen-doped nanoporous carbon (N-NPC) via a hydrothermal reaction using urea as the nitrogen source. The obtained N-NPC possessed a large surface area (1075 m² g⁻¹), narrow pore-size distribution (1–2 nm) and nitrogen content reaching 2.23 wt%. Using N-NPC with Nafion to modify a glassy carbon electrode, a highly sensitive electrochemical sensor was fabricated for the determination of Pb(ii) in aqueous solutions with differential pulse anodic stripping voltammetry (DPASV). The peak current of Pb(ii) showed linearity over concentrations from 2.0 to 120 μg L⁻¹ and the detection limit (S/N = 3) was estimated to be 0.7 μg L⁻¹ for Pb(ii), which was 15-fold lower than the guideline value of drinking water given by the World Health Organization (WHO). The experimental data indicated that this easy and low-cost method is an accurate and fast method for the detection of trace Pb(ii).

Almond shell-based charcoal was prepared by carbonizing almond shells in a nitrogen atmosphere.     

Dual-emission ratio fluorescence for selective and sensitive detection of ferric ions and ascorbic acid based on one-pot synthesis of glutathione protected gold nanoclusters

A fluorometric method was proposed for the determination of Fe³⁺ and ascorbic acid (AA) based on blue and red dual fluorescence emissions of glutathione (GSH) stabilized-gold nanoclusters (AuNCs). AuNCs were synthesized from GSH and tetrachloroauric acid. The fluorescence peaks of AuNCs were at 425 nm and 585 nm, respectively. In the presence of Fe³⁺, the fluorescence peak at 425 nm can be enhanced and that at 585 nm can be quenched. There is a good linear relationship between the fluorescence intensity ratio for the 425 and 585 nm peaks (F₄₂₅/F₅₈₅) and the concentration of Fe³⁺ in the range of 0.75–125 μM. However, when AA was added to the AuNCs–Fe³⁺ system, the value of F₄₂₅/F₅₈₅ decreased consistently with the concentration of AA in the range of 0.25–35 μM. The limit of detection for Fe³⁺ and AA was 227 and 75.8 nM, respectively. The interaction between AuNCs and Fe³⁺ can induce the ligand–metal charge transfer (LMCT) effect leading to the fluorescence increment at 425 nm, while AA can reduce Fe³⁺ to Fe²⁺. The production of Fe²⁺ can not enhance or quench the fluorescence of AuNCs. By comparison with previous literature, the AuNCs prepared here show two fluorescence peaks without additional fluorescence labels. Furthermore, the method was successfully applied in the determination of Fe³⁺ and AA in some real samples, such as water, human serum and tablets.

A fluorometric method was proposed for the determination of Fe³⁺ and ascorbic acid (AA) based on blue and red dual fluorescence emissions of glutathione (GSH) stabilized-gold nanoclusters (AuNCs).     

Synthesis and evaluation of the SERS effect of Fe3O4–Ag Janus composite materials for separable,highly sensitive substrates

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method. The optimal growth process was determined by investigating the relationship between the particle morphologies and reaction time. Magnetic and Raman spectroscopic measurements showed that the as-synthesized Janus composites have both good magnetic response and significant surface-enhanced Raman scattering (SERS) effects, as well as reproducibility. The calculated Raman enhancement factor reached an unprecedented magnitude of 10⁹ compared with the values of other Fe₃O₄–Ag compounds. Furthermore, the SERS effect was exhibited even at a concentration of probe molecules as low as 10⁻¹³ M. This demonstrates that the as-synthesized Fe₃O₄–Ag Janus composite particles have promise for application as separable, highly sensitive SERS substrates.

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method.     

An easy synthesis of nitrogen and phosphorus co-doped carbon dots as a probe for chloramphenicol

Heteroatom doping in carbon dots (CDs) was found to be an efficient way to regulate the structure of electronic energy levels and enhance the fluorescence characteristics of CDs. Nevertheless, most reported fabrication processes of heteroatom-doped CDs are rigorous and complex. Herein, a facile and novel strategy was developed to rapidly prepare nitrogen and phosphorus co-doped CDs (N,P-CDs) using acetic acid as the carbon source, and arginine, 1,2-ethylenediamine (EDA) and diphosphorus pentoxide as the dopants, respectively. The optical, morphological and structural characterizations of the synthesized N,P-CDs were investigated via UV and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, TEM, and FT-IR spectroscopy. The N,P-CDs display outstanding fluorescence stability under high ionic strength (1.6 M KCl), and long time UV irradiation, indicating that they can be used as favorable candidates for fluorescent probes. The fluorescence of N,P-CDs was selectively quenched by chloramphenicol (CAP) with a short response time. The linear range of the response to CAP was from 0.8 to 70 μM with a limit of detection of 0.36 μM (S/N = 3). Notably, the fabricated N,P-CDs were employed for the highly selective and sensitive detection of CAP in milk samples, indicating their potential applications in biologically related areas.

The spontaneous synthesis of nitrogen and phosphorus co-doped carbon dots was reported, and they were used as a probe for chloramphenicol.     

Novel single excitation dual-emission carbon dots for colorimetric and ratiometric fluorescent dual mode detection of Cu2+ and Al3+ ions

In this study, dual-emission carbon dots (D-CDs) are synthesized via a simple one-step solvothermal treatment of red tea. The obtained D-CDs are characterized by XPS, IR, TEM, XRD, fluorescence and UV-vis spectroscopy techniques. It is found that D-CDs present a strong red fluorescence emission peak at 671 nm and weak blue fluorescence emission peak at 478 nm under the excitation wavelength of 410 nm. The unique dual-emission properties of D-CDs provide great opportunities in ratiometric fluorescence sensing applications. The results show that Cu²⁺ ions can quench the fluorescence of the red emission band of D-CDs effectively, resulting in the disappearance of red fluorescence ultimately. Upon the addition of Al³⁺ ions, the fluorescence of blue emission band at 478 nm grows apparently, and the fluorescence color transforms gradually from red to orange, then to yellow-green. Based on these findings, a novel ratiometric fluorescence and colorimetric dual mode nanosensor is developed for simultaneous detection of Cu²⁺ and Al³⁺ ions. Regarding Cu²⁺ ions, the fluorescent detection linear range is 0.1–50 μM with detection limit of 0.1 μM, and the colorimetric detection limit is estimated as 25 μM. With regard to Al³⁺ ions, the fluorescent detection linear range is 0–20 μM and 25–100 μM with detection limit of 0.5 μM, and the colorimetric detection limit is 20 μM. Furthermore, the fluorescence response mechanisms of Cu²⁺ and Al³⁺ ions were discussed detailed. To the best of our current knowledge, this will be the first research work on the simultaneous determination of Cu²⁺ and Al³⁺ using D-CDs as fluorescent probes.

D-CDs with strong red emission and weak blue emission as an effective colorimetric and ratiometric fluorescence sensing probe are employed to realize the simultaneous detection of Cu²⁺ and Al³⁺ ions without any interference effect.     

GSH-doped GQDs using citric acid rich-lime oil extract for highly selective and sensitive determination and discrimination of Fe3+ and Fe2+ in the presence of H2O2 by a fluorescence “turn-off” sensor

Synthesis and characterization of graphene quantum dots (GQDs) simultaneously doped with 1% glutathione (GSH-GQDs) by pyrolysis using citric acid rich-lime oil extract as a starting material. The excitation wavelength (λ_max = 337 nm) of the obtained GSH-GQD solution is blue shifted from that of bare GQDs (λ_max = 345 nm), with the same emission wavelength (λ_max = 430 nm) indicating differences in the desired N and S matrices decorating the carbon based nanoparticles, without any background effect of both ionic strength and masking agent. For highly Fe³⁺-sensitive detection under optimum conditions, acetate buffer at pH 4.0 in the presence of 50 μM H₂O₂, the linearity range was 1.0–150 μM (R² = 0.9984), giving its calibration curve: y = 34.934x + 169.61. The LOD and LOQ were found to be 0.10 and 0.34 μM, respectively. The method’s precisions expressed in terms of RSDs for repeatability (n = 3 × 3 for intra-day analysis) were 2.03 and 3.17% and for reproducibility (n = 5 × 3 for inter-day analysis) were 3.11 and 4.55% for Fe²⁺ and Fe³⁺, respectively. The recoveries of the method expressed as the mean percentage (n = 3) were found in the ranges of 100.1–104.1 and 98.08–102.7% for Fe²⁺ and Fe³⁺, respectively. The proposed method was then implemented satisfactorily for trace determination of iron speciation in drinking water.

Synthesis and characterization of graphene quantum dots (GQDs) simultaneously doped with 1% glutathione (GSH-GQDs) by pyrolysis using citric acid rich-lime oil extract as a starting material.     

The aggregation of Fe3+ and their d–d radiative transitions in ZnSe:Fe3+ nanobelts by CVD growth

Transition metal (TM) doped II–VI semiconductors have attracted great attention due to their luminescence and diluted magnetism. In this study, the Fe³⁺-doped ZnSe nanobelts (NBs) were grown by a facile CVD method. The surface morphology observed via SEM is smooth and clean and the elemental composition measured via EDS confirms that the Fe³⁺ ions were incorporated into ZnSe NBs successfully. The micro-Raman scattering spectra demonstrate that the as-prepared NBs have the zinc blende structure. Furthermore, the Raman spectra of the Fe³⁺-doped NBs were compared with those of pure and Fe²⁺-doped reference samples. The former with a higher signal-to-noise ratio, an enhanced 2LO mode, a stronger LO mode redshift and a larger intensity ratio of LO/TO mode as well as the lower acoustic phonon modes confirms the better crystallization and the stronger electron–phonon coupling on Fe³⁺-incorporation. The emission of single Fe³⁺ ion, assigned to the ⁴T₁ → ⁶A₁ transition, was observed at about 570 nm. Moreover, increasing the doping concentration of Fe³⁺ ions caused the formation of different Fe–Fe coupled pairs in the lattice, which emitted light at about 530–555 nm for an antiferromagnetic-coupled pair, possibly due to the stacking faults and at about 620–670 nm for a ferromagnetic-coupled pair.

Transition metal (TM) doped II–VI semiconductors have attracted great attention due to their luminescence and diluted magnetism.     

Synthesis and bioimaging of a BODIPY-based fluorescence quenching probe for Fe3+

Iron is the main substance for maintaining life. Real-time determination of ferric ion (Fe³⁺) in living cells is of great significance for understanding the relationship of Fe³⁺ concentration changes with various physiological and pathological processes. Fluorescent probes are suitable for the detection of trace metal ions in cells due to their low toxicity and high sensitivity. In this work, a boron-dipyrromethene-based fluorescent probe (BODIPY-CL) for selective detection of Fe³⁺ was synthesized. The fluorescence emission of BODIPY-CL was determined at 516 nm. In a pH range of 1 to 10, the probe BODIPY-CL exhibits a quenching response to Fe³⁺. Meanwhile, BODIPY-CL showed a highly selective response to Fe³⁺ compared with 16 kinds of metal ions. The stoichiometry ratio of BODIPY-CL bound to Fe³⁺ was nearly 2 : 1. The fluorescence quenching response obtained by the sensor was linear with the Fe³⁺ concentration in the range of 0–400 μM, and the detection limit was 2.9 μM. BODIPY-CL was successfully applied to image Fe³⁺ in cells. This study provides a promising fluorescent imaging probe for further research on the physiological and pathological effects of Fe³⁺.

A quenched fluorescence probe sensitive to Fe³⁺ ions was synthesized. The probe was successfully used to detect Fe³⁺ in living organisms.     

Selective fluorescent sensing of LMOFs constructed from tri(4-pyridylphenyl)amine ligand

Three new luminescent metal–organic frameworks (LMOFs), [Zn(tppa)(ndc)]_n (1), [Cd(tppa)(oba)]_n (2), [Zn₂(tppa)(bpdc)₂]_n (3) (tppa = tri(4-pyridylphenyl)amine, ndc = 1,4-naphthalenedicarboxylic acid, oba = 4,4′-oxydibenzoic acid, bpdc = 4,4′-biphenyldicarboxylic acid) have been synthesized by solvothermal method. Complexes 1 and 2 are 2-D two-fold interpenetrating structures, aligning into a 3-D structure through C–H⋯π stacking interactions, while 3 is a 5-fold interpenetrating three-dimensional structure. The internal quantum yields (IQYs) of complexes 1–3 are 32.7%, 45.7% and 24.0% (λ_ex = 365 nm), separately. Furthermore, all the complexes show different luminescence signal changes towards aromatic volatile organic compounds (AVOCs). Complex 1 exhibits a high sensitivity in the detection of both Fe³⁺ and Cr³⁺ with large quenching coefficients of K_sv 2.57 × 10⁴ M⁻¹ and 2.96 × 10⁴ M⁻¹, respectively. All these results demonstrated potential applications in chemical sensing.

Three new LMOFs, complexes 1–3, have been solvothermally synthesized. 1 and 2 are 2-D structures, whiles 3 is a 3-D structure. And 1 exhibits in detecting Fe³⁺ and Cr³⁺. All of them have potential applications in chemical sensing.     

Hollow terbium metal–organic-framework spheres: preparation and their performance in Fe3+ detection

Hollow metal–organic framework (MOF) micro/nanostructures have been attracting a great amount of research interest in recent years. However, the synthesis of hollow metal–organic frameworks (MOFs) is a great challenge. In this paper, by using 1,3,5-benzenetricarboxylic acid (H₃BTC) as the organic ligand and 2,5-thiophenedicarboxylic acid (H₂TDC) as the competitive ligand and protective agent, hollow terbium MOFs (Tb-MOFs) spheres were synthesized by a one-pot solvothermal method. By comparing the morphology of Tb-MOFs in the presence and absence of H₂TDC, it is found that H₂TDC plays a key role in the formation of the hollow spherical structure. Single crystal analyses and element analysis confirm that H₂TDC is not involved in the coordination with Tb³⁺. Interestingly, Tb-MOFs can be used as the luminescent probes for Fe³⁺ recognition in aqueous and N,N-dimethylformamide (DMF) solutions. In aqueous solution, the quenching constant (K_SV) is 5.8 × 10⁻⁴ M⁻¹, and the limit of detection (LOD) is 2.05 μM. In DMF, the K_SV and LOD are 9.5 × 10⁻⁴ M⁻¹ and 0.80 μM, respectively. The sensing mechanism is that the excitation energy absorption of Fe³⁺ ions reduces the energy transfer efficiency from the ligand to Tb³⁺ ions.

(a) Pictures of Tb-MOFs suspension (left) and Fe³⁺ (right) under 365 nm illumination. (b) Pictures of Fe³⁺ with (left) and without (right) Tb-MOFs. (c) Pictures of Tb-MOFs powder before (left) and after (right) Fe³⁺ adsorption.     

Highly sensitive naphthalimide based Schiff base for the fluorimetric detection of Fe3+

A simple 1,8-naphthalimide based Schiff base probe (E)-6-((4-(diethylamino)-2-hydroxybenzylidene)amino)-2-(2-morpholinoethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NDSM) has been designed and synthesized for the specific detection of Fe³⁺ based on a fluorimetric mode. The absorbance of NDSM at 360 nm increased significantly in acetonitrile : water (7 : 3, v/v) medium only in the presence of Fe³⁺ ions with a visible colour change from yellow to golden yellow. Likewise, fluorescence emission intensity at 531 nm was almost wholly quenched in the presence of Fe³⁺. However, other competitive ions influenced insignificantly or did not affect the optical properties of NDSM. Lysosome targetability was expected from NDSM due to the installation of a basic morpholine unit. The LOD was found to be 0.8 μM with a response time of seconds. The fluorescence reversibility of NDSM + Fe³⁺ was established with complexing agent EDTA. Fe³⁺ influences the optical properties of NDSM by complexing with it, which blocks C N isomerization in addition to the ICT mechanism. The real-time application of Fe³⁺ was demonstrated in test paper-based detection, by the construction of a molecular logic gate, quantification of Fe³⁺ in water samples and fluorescence imaging of Fe³⁺.

A simple 1,8-naphthalimide based Schiff base probe NDSM has been designed and synthesized for the specific detection of Fe³⁺ based on a fluorimetric mode.     

Highly selective fluorescent turn-on–off sensing of OH−, Al3+ and Fe3+ ions by tuning ESIPT in metal organic frameworks and mitochondria targeted bio-imaging

Herein we report a multifunctional high performance metal organic framework (Zn-DHNDC MOF) based chemosensor that displays an exceptional excited state intramolecular proton transfer (ESIPT) tuned fluorescence turn-on–off response for OH⁻, Al³⁺ and Fe³⁺ ions along with mitochondria targeted bio-imaging. Properly tuning ESIPT as well as the hydroxyl group (–OH) allows Zn-DHNDC MOF to optimize and establish chelation enhanced fluorescence (CHEF) and chelation enhanced quenching (CHEQ) based sensing mechanisms. The MOF benefits from acid-base interactions with the ions which generate a turn-on bluish green fluorescence (λ_Em 492 nm) for OH⁻, an intense turn-on green fluorescence (λ_Em 528 nm) for Al³⁺ and a turn-off fluorescence quenching response for Fe³⁺ ions. The aromatic –OH group indeed plays its part in triggering CHEF and CHEQ processes responsible for the turn-on-off events. Low limits of detection (48 nM of OH⁻, 95 nM for Al³⁺, 33 nM for Fe³⁺ ions), high recyclability and fast response time (8 seconds) further assist the MOF to implement an accurate quantitative sensing strategy for OH⁻, Al³⁺ and Fe³⁺ ions. The study further demonstrates the MOF''s behaviour in cellular medium by subjecting it to live cell confocal microscopy. Along with a bio-compatible nature the MOF exhibited successful accumulation inside the mitochondria of MCF7 cancer cells, which defines it as a significant bio-marker. Therefore the present work successfully represents the multidisciplinary nature of Zn-DHNDC MOFs, primarily in sensing and biomedical studies.

ESIPT tuned fluorescence sensing of OH⁻, Al³⁺ and Fe³⁺ ions and mitochondria targeted bio-imaging by a Zn-DHNDC MOF.     

Development of a fused imidazo[1,2-a]pyridine based fluorescent probe for Fe3+ and Hg2+ in aqueous media and HeLa cells

A new fluorescent sensor 5 based on a fused imidazopyridine scaffold has been designed and synthesized via cascade cyclization. The reaction features the formation of three different C–N bonds in sequence. Imidazopyridine based fluorescent probe 5 exhibits highly sensitive and selective fluorescent sensing for Fe³⁺(‘turn-on’) and Hg²⁺(‘turn-off’). The excellent selectivity of imidazopyridine for Fe³⁺/Hg²⁺ was not hampered in the presence of any of the competing cations. The limit of detection (LOD) of 5 toward Fe³⁺ and Hg²⁺ has been estimated to be 4.0 ppb and 1.0 ppb, respectively, with a good linear relationship (R² = 0.99). Notably, 5 selectively detects Fe³⁺/Hg²⁺ through fluorescence enhancement signalling both in vitro and in HeLa cells.

A new fluorescent sensor 5 having fused imidazopyridine scaffold has been synthesized via cascade cyclization. It exhibits highly sensitive and selective detection of Fe³⁺ (‘turn-on’) and Hg²⁺ (‘turn-off’) in vitro and in HeLa cells.     

Facile solvothermal preparation of Fe3O4–Ag nanocomposite with excellent catalytic performance

Functional nanocomposites demonstrate excellent comprehensive properties and outstanding characteristics for numerous applications. Magnetic nanocomposites are an important type of composite materials, due to their applications in optics, medicine and catalysis. In this report, a new Fe₃O₄-loaded silver (Fe₃O₄–Ag) nanocomposite has been successfully synthesized via a simple solvothermal method and in situ growth of silver nanowires. The silver nanowires were prepared via the reduction of silver vanadate with the addition of uniformly dispersed Fe₃O₄ nanoparticles. Structural and morphological characterizations of the obtained Fe₃O₄–Ag nanocomposite were carried out using many characterization methods. As a new composite catalyst, the synthesized magnetic Fe₃O₄–Ag nanocomposite displayed a high utilization rate of catalytically active sites in catalytic reaction medium and showed good separation and recovery using an external magnetic field. The facile preparation and good catalytic performance of this Fe₃O₄–Ag nanocomposite material demonstrate its potential applications in catalytic treatment and composite materials.

A new Fe₃O₄–Ag nanocomposite was prepared via solvothermal method, demonstrating potential application in catalytic degradation of wastewater treatment and composite materials.     

Synthesis of BINOL–xylose-conjugates as “Turn-off” fluorescent receptors for Fe3+ and secondary recognition of cysteine by their complexes

A novel chiral fluorescence “turn-off” sensor was synthesised using the click reaction. The sensor was a BINOL–xylose derivative, modified at the 2-position and linked by 1,2,3-triazole. It was structurally characterized by ¹HNMR, ¹³CNMR, ESI-MS and IR analysis. The selectivity of R-β-d-2 in methanol solution has been studied. Among the 19 transition metal ions, alkaline metal ions and alkaline earth metal ions studied, R-β-d-2 had a selective fluorescence quenching reaction for Fe³⁺. The detection limit of R-β-d-2 for Fe³⁺ was 0.91 μmol L⁻¹. Complexation between R-β-d-2 and Fe³⁺ was investigated by ESI-MS and ¹HNMR. The stoichiometric ratio of R-β-d-2 was 1 : 1. In addition, the R-β-d-2–Fe³⁺ complex was titrated with 20 naturally occurring amino acids and Hcy with GSH. It was found that the complex R-β-d-2–Fe³⁺ had a secondary recognition effect on Cys by switching to fluorescence.

A fluorescence sensor of BINOL–xylose derivative was synthesized, which could only detect Fe³⁺ by 1 + 1 complex with high selectivity and sensitivity. The complex of the derivative with Fe³⁺ was found to perform secondary recognition of cysteine.     

Synthesis and magnetic properties of sub-nanosized iron carbides on a carbon support

Iron carbide clusters with near-sub-nanometer size have been synthesized by employing a tetraphenylmethane-cored phenylazomethine dendrimer generation 4 (TPM-DPAG4) as a molecular template. Magnetic measurements reveal that these iron carbide clusters exhibit a magnetization–field hysteresis loop at 300 K. The data indicate that these iron carbide clusters are ferromagnets at room temperature.

This study reports the synthesis and ferromagnetism of iron carbide clusters with near-subnanometer size by employing a dendrimer template and carbothermal hydrogen reduction (CHR).

Iron carbide is a well-established material that is typically generated during the steelmaking process. Research into the phase diagram of the Fe–C system was conducted as early as the 1890s.¹ According to this phase diagram, iron and carbon atoms can be mixed in arbitrary proportions up to 0.095 atom% of C at temperatures below 1000 K; above this ratio iron carbide cementite (Fe₃C) is formed.² As with metallic iron, iron carbides are also known to exhibit ferromagnetism;^3,4 therefore, there have been many studies reported on the ferromagnetism of bulk iron carbides and iron carbide nanoparticles.^5–19 The size effect in nanomaterials is also of particular interest because the properties of the bulk materials can be significantly changed. For example, melting-point depression,²⁰ catalyst activation,²¹ and the alloying of non-mixable metals²² have been reported to occur as the particle size decreases into the nanosize range. We have recently reported atomicity-dependent changes in the catalytic activity^23,24 and size-dependent phase transformations of near-sub-nanometer particles.²⁵ The properties of many substances are thus sensitively affected by particle size, particularly in the near-sub-nanosize range. In this context, the smallest iron carbide nanoparticles reported to date are as small as ca. 2 nm,^7,16 aside from the gas phase experiments^26,27 and the theoretical studies.^28–32 In these cases, the iron carbide nanoparticles exhibit superparamagnetism, i.e., they do not act as magnets at ambient temperature. However, sub-nanosized iron carbide particles have remained elusive to date. In the present study, we have synthesized near-sub-nanometer iron carbide particles/clusters, and these iron carbide clusters are ferromagnets, even at room temperature, thereby countering superparamagnetism. Bulk iron carbide is an old material; however, the iron carbide clusters synthesized in this work are the smallest room temperature magnets reported to date. Fig. 1 shows the strategy employed for the synthesis of near sub-nanometer-sized iron carbide clusters. The macromolecular tetraphenylmethane-cored dendritic phenylazomethine dendrimer generation 4 (TPM-DPAG4) was used as a molecular template. This DPA-type dendrimer coordinates to metal ions in solution via its imine sites, and complexation proceeds stepwise from the center of the dendrimer to its periphery due to its basicity gradient.^33–37 Stepwise complexation was confirmed in the present study by UV-Vis titrations. Upon the addition of FeCl₃ to a solution of TPM-DPAG4, spectral changes and shifts in the isosbestic point were observed (Fig. S1^†); these changes reached saturation after the combined addition of 60 eq. of FeCl₃. Different isosbestic points were observed in the ranges of 0–4, 6–12, 16–28, and 32–60 eq., respectively, which is consistent with the number of imines at each type of site and reflects the stepwise complexation from the central to the peripheral sites. The in situ-prepared dendrimer complexes, i.e., TPM-DPAG4 with 4, 12, 28, or 60 eq. of FeCl₃ incorporated were then adsorbed onto a graphitic carbon support (graphitized mesoporous carbon: GMC). Carbothermal hydrogen reduction (CHR), which is a synthetic method used to obtain metal carbides, was subsequently applied.^25,38,39 After CHR at 773 K for 30 min, the samples (Fe₁₂/C, Fe₂₈/C, and Fe₆₀/C) were examined using transmission electron microscopy (TEM), and the results are shown in Fig. 2 and S2.^† There are several reports for TEM observations of iron carbide nanoparticles larger than 2 nm diameter without atomic-resolution.^5–19 Very fine particles dispersed over the carbon support were observed as blurry black dots in the TEM images. The mean particle diameter and standard deviation of the size distribution were estimated to be 0.9 ± 0.2 nm (Fe₁₂/C), 1.0 ± 0.3 nm (Fe₂₈/C), and 1.3 ± 0.3 nm (Fe₆₀/C), respectively. The average particle size consistently increased with the FeCl₃ content in the TPM-DPAG4 template. These samples represent the first examples of near-sub-nanometer-sized iron carbide particles. However, we could not observe any individual particles in the Fe₄/C sample, because the particle size was too small. In this case, the particle size was estimated to be ca. 0.6 nm using the tetra-nuclear cluster model of the [Fe₄C(CO)₁₂]²⁻ carbidocarbonyl complex reported by Boehme et al. (Fig. S3a^†)⁴⁰ as well as the theoretical studies.^28–32 It should be noted that atomic-resolution images that would project the clusters could not be obtained, because these samples exhibit ferromagnetism, even at room temperature (vide infra).Open in a separate windowFig. 1Chemical structure of the TPM-DPAG4 and illustration of metal ions assembly (4, 12, 28, 60 eq.).Open in a separate windowFig. 2TEM images of Fe₁₂/C, Fe₂₈/C, and Fe₆₀/C after 30 min of CHR at 773 K.Powder X-ray diffraction (PXRD) analysis cannot be applied to the characterization of such sub-nanosized particles on solid supports, as they do not adopt any long-range-ordering crystal structure. On the other hand, X-ray absorption fine structure (XAFS) is a powerful tool to clarify the local structure around the metal atoms.⁴¹ We found that the X-ray absorption near edge structure (XANES) spectra of Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR are very similar to those of metallic iron (Fe foil) and Fe₃C, whereas they are substantially different from those of iron oxides such as Fe₃O₄ and α-Fe₂O₃ as well as from that of the FeCl₃ starting material (Fig. S4^†). Therefore, it can be concluded that these samples are not oxides. XANES spectrum of Fe₃C and metallic iron can clearly be distinguished in their first derivatives form (Fig. 3). Metallic iron and Fe₃C have a pre-edge peak in common at ca. 7111 eV (3s → 4d transitions). Metallic iron exhibits two maxima in the range of 7115–7130 eV (3s → 4p transitions), while Fe₃C exhibits one maximum and several shoulders in this region. The spectra of Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR had a pre-edge peak at ca. 7111 eV, together with a maximum peak in the 7115–7130 eV region, which indicates the iron carbide nature. Therefore, it can be concluded that Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C are iron carbides rather than metals. The white-line peak was slightly shifted to the higher energy side with downsizing (Fig. 3b), i.e., 7129.4 eV (Fe₃C), 7130.2 eV (Fe₆₀/C), 7130.1 eV (Fe₂₈/C), 7131.2 eV (Fe₁₂/C), and 7131.4 eV (Fe₄/C). The experimental error was estimated to be ±0.3 eV based on the applied energy resolution. This shift tendency supported that the cluster samples (Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C) are very fine particles with high specific surface. In addition, the assignment as iron carbides is decisively supported by their Curie temperatures (T_C), which were measured to be 483–488 K (Fig. 4). These T_C values are comparable to that of Fe₃C (483 K, Fig. S5 and S6^†),⁹ which suggests that the ferromagnetic interactions originate from iron carbides. The Curie temperature of Fe₃C is far from those of metallic iron (1043 K)³ and iron oxides e.g. Fe₃O₄ (850 K) and γ-Fe₂O₃ (820–986 K).⁴² It should also be noted that Fe₃C forms a complicated crystal structure that involves nine types of Fe–Fe bonds (2.455–2.714 Å; Fig. S3b^†).^43,44 The presence of the corresponding Fe–Fe bonds in Fe₆₀/C was suggested by extended X-ray absorption fine structure (EXAFS) measurements conducted in transmission mode (Fig. S7^†). The Curie temperature for Fe₃C mainly represents the average of the direct exchange interactions between the Fe–Fe bonds, similar to that in amorphous ferromagnets such as the Fe–C–P system,^45,46 and thus, T_C would be considered not to show a significant size dependence.Open in a separate windowFig. 3Fe K-edge XANES spectra. (a) First derivatives of normalized XANES spectra for Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR at 773 K for 30 min, together with those for Fe foil (metallic iron), Fe₃C, Fe₃O₄, and α-Fe₂O₃. The spectra for Fe₂₈/C, Fe₁₂/C, and Fe₄/C were recorded in fluorescence mode, whereas the others were recorded in transmission mode. (b) Magnifications around the white-line peak. The experimental error was estimated to be ±0.3 eV based on the applied energy resolution.Open in a separate windowFig. 4Temperature-dependent magnetization curves for (a) Fe₆₀/C, (b) Fe₂₈/C, (c) Fe₁₂/C, and (d) Fe₄/C obtained by application of a magnetic field (5000 Oe) and measurement of the magnetization in increments of 10 K (300–420 K) or 5 K (420–600 K). The blue lines are smoothed trend lines. The Curie point (T_C) was determined from the maximum of the second derivative (insets) and calibrated using T_C = 483 K for Fe₃C.⁹ The error in the maxima of the second derivatives was estimated to be 5 K. Fig. 5 shows magnetization–field (M–H) loops for the iron carbide clusters, and the magnetic data are summarized in Table S1.^† The M per the sample weight data are shown in Fig. S8–S12.^† The four cluster samples show hystereses in their M–H loops at 1.9 K (Fig. 5a), which indicates that they are ferromagnets with an associated coercivity (H_c). The H_c value increased with a decrease in the cluster size, i.e., 603 Oe (Fe₆₀/C), 939 Oe (Fe₂₈/C), 1856 Oe (Fe₁₂/C), and 2697 Oe (Fe₄/C). In contrast, bulk iron carbide cementite (Fe₃C) with an average crystal size of 39 nm has a smaller hysteresis with H_c values of 166 and 21 Oe at 1.9 and 300 K, respectively (Fig. S8^†). The magnetic behavior of bulk Fe₃C indicates ferromagnetism with a multi-magnetic-domain structure.³ On the contrary, iron carbide nanoparticles have been reported to exhibit more pronounced hysteresis at room temperature than bulk Fe₃C, with H_c values of 700 Oe (15 nm) and 544 Oe (14.1 ± 0.8 nm) by Grimes et al.⁵ and Hou et al.,⁶ respectively, which suggests a single-magnetic-domain structure. Therefore, the smaller iron carbide clusters in this study are considered to have a single-magnetic-domain structure. The increase in coercivity with the decrease in single-magnetic-domain particle size has been reported by Lartigue et al. for iron carbide nanoparticles with sizes of 15.1 nm (H_c = 331 Oe), 7.4 nm (H_c = 405 Oe), 5.5 nm (H_c = 625 Oe), and 2.8 nm (H_c = 1009 Oe) at 2.5 K.⁷ On the other hand, the iron carbide clusters exhibit hysteresis loops at 300 K (Fig. 5b), i.e., H_c = 140 Oe (Fe₆₀/C), 163 Oe (Fe₂₈/C), 367 Oe (Fe₁₂/C), and 666 Oe (Fe₄/C), which indicates that they are ferromagnets, even at room temperature. Clusters or near-sub-nanosize particles generally exhibit superparamagnetism with a complete loss of coercivity at room temperature.³τ_N ∝ exp(E_a/k_BT)1E_a ≃ K_effV2Open in a separate windowFig. 5Magnetization–field (M–H) loops for Fe₆₀/C (magenta), Fe₂₈/C (brown), Fe₁₂/C (green), and Fe₄/C (blue) at (a) 1.9 K and (b) 300 K. Magnetization (M) was normalized with respect to the saturation magnetization (M_s). The inset shows the magnification in the region near zero field. Eqn (1) is the Néel–Arrhenius equation,⁴⁷ where τ_N, E_a, k_B, T, K_eff, and V are the Néel relaxation time, magnetic anisotropy energy, Boltzmann constant, temperature, effective magnetic anisotropy constant, and volume of a single-magnetic-domain particle, respectively. The term for the angle between the magnetic moment and the easy magnetic axis was not introduced (eqn (2)), because these were powder samples. Therefore, superparamagnetism emerges with a decrease in size (V) because E_a becomes comparable to the thermal energy (k_BT). Lartigue et al. have reported superparamagnetism for iron carbide nanoparticles with sizes less than 5.5 nm.⁷ Fig. S13^† shows field-cooling (FC) and zero-field-cooling (ZFC) magnetization curves used to determine the blocking temperature (T_B) at which the magnets completely lose their coercivity. Fe₃C shows a T_B of 467 K which is near the Curie point (483 K). On the other hand, the T_B of Fe₆₀/C is clearly lower (ca. 385 K) than the Curie point, which was attributed to the influence of superparamagnetism in light of the results of Lartigue et al. In contrast, Fe₂₈/C has higher T_B values close to the Curie point at 473 K. This behavior is contrary to superparamagnetism and cannot be explained without increased effective magnetic anisotropy (K_eff). The interactions between the iron carbide clusters and the graphitic carbon surface may be a mechanism to afford large K_eff, because the ratio of the interacting Fe atoms increases by decreasing size. The oxidation of Fe₄/C in air at 553 K for 30 min significantly decreased the magnetization and coercivity (Fig. S14^†). Carbides can be the magnets at sub-nano scale, while oxides would not. Due to the measurement sensitivity limit and noise, the T_Bs of Fe₁₂/C and Fe₄/C were roughly estimated to be 410–470 K and 350–470 K, respectively, which indicates that their T_Bs were at least above room temperature. It should also be noted here that the magnetic moment per Fe atom of these cluster samples (1.0–2.3 μ_B atom_Fe⁻¹) was almost identical to that of Fe₃C (1.5 μ_B atom_Fe⁻¹), regardless of the Fe content (wt%) over an order of magnitude (Table S1^†). The variation in the magnetic moment may involve not only experimental errors, but also atomicity. Becker et al. reported that Fe clusters exhibit an atomicity-dependent variation in their magnetic moment in the gas phase, especially below 100 atoms.⁴⁸ Additionally, the density functional theory studies have reported that the iron carbide clusters show the magnetic moment of ca. 1–3.5 μ_B atom_Fe⁻¹,^30–32 which is consistent with those in this study. Therefore, with consideration of the magnetic moment and the Curie temperature, it was concluded that the magnetic behavior of the iron carbide cluster samples is derived from the carbides themselves, and not from impurities. The reproducibility of the M–H hysteresis loop at 300 K for Fe₄/C was certainly confirmed including another batch sample (sample B: Fig. S15^†). It was also confirmed that a blank sample (GMC) showed diamagnetism measured at both 1.9 and 300 K (raw M data shown in Fig. S16–S23^†).Nanoparticle magnets have a single magnetic-domain structure and exhibit hysteresis at room temperature; however, they lose this hysteresis upon downsizing by superparamagnetism. There have been no reports of sub-nanoparticle magnets (diameter: ∼1 nm or less) that exhibit coercivity above room temperature;^3–19,49 neither for e.g. Fe–Pt bimetallic nanoparticles⁵⁰ nor iron oxide nanoparticles.⁵¹ The iron carbide clusters in this study are unique magnets that are different from both nanoparticle magnets. They do not have a long-range-ordering crystal structure such as nanoparticles and bulk substances on account of their sub-nanometer size. The iron carbide clusters in this study were carefully characterized by XAFS (Fig. 3) as well as by the Curie temperature (Fig. 4). The magnetic measurements (Fig. 5) revealed that the iron carbide clusters represent room-temperature magnets. Therefore, the iron carbide clusters discussed in this study can be regarded as a new class of magnets, i.e., sub-nano magnets.This work has synthesized the first examples of sub-nanosized iron carbides on a graphitic carbon support. These iron carbide clusters act as magnets at room temperature. This study would open up the new research field of sub-nano magnets.     

A stimuli responsive triplet–triplet annihilation upconversion system and its application as a ratiometric sensor for Fe3+ ions

A ratiometric fluorescent sensor for the detection of Fe³⁺ ions is achieved based on triplet–triplet annihilation upconversion (TTA-UC) luminescence. A new anthracene derivative (named as DHTPA) is designed and synthesized and reveals similar optical properties to 9,10-diphenylanthracene (DPA) and is used as a stimuli responsive annihilator in a TTA-UC system due to its complexation ability. As a result, the UC emission can be significantly quenched by Fe³⁺ ions, while the phosphorescence (PL) emission of sensitizer palladium(ii) octaetylporphyrin (PdOEP) remains nearly constant, which makes the PL signal an appropriate internal reference for the UC signal. The UC and ratio signals (I_UC/I_PL) both reveal a good linear relationship with Fe³⁺ ion concentration, which for the first time makes the TTA-UC system a perfect ratiometric sensor for Fe³⁺ ion detection. This sensing method will open a novel avenue to achieve ratiometric sensors in chemical and biological fields.

A ratiometric fluorescent sensor for detection of Fe³⁺ is achieved based on a triplet–triplet annihilation upconversion (TTA-UC) system with a responsive annihilator.     

Pt–Nix alloy nanoparticles: a new strategy for cocatalyst design on a CdS surface for photo-catalytic hydrogen generation

Solar photocatalytic water splitting for the production of hydrogen has been a core aspect for decades. A highly active and durable photocatalyst is crucial for the success of the renewable hydrogen economy. To date, the development of highly effective photocatalysts has been seen by the contemporary research community as a grand challenge. Thus, herein we put forward a sincere attempt to use a Pt–Ni_x alloy nanoparticle (NP) cocatalyst loaded CdS photocatalyst ((Pt–Ni_x)/CdS) for photocatalytic hydrogen production under visible light. The Pt–Ni_x alloy NP cocatalyst was synthesized using a one-pot solvothermal method. The cocatalyst nanoparticles were deposited onto the surface of CdS, forming a Pt–Ni_x/CdS photocatalyst. Photocatalytic hydrogen production was carried out using a 300 W Xe light equipped with a 420 nm cut-off filter. The H₂ evolution rate of the Pt–Ni₃/CdS photocatalyst can reach a value as high as 48.96 mmol h⁻¹ g⁻¹ catalyst, with a quantum efficiency of 44.0% at 420 nm. The experimental results indicate that this Pt–Ni₃/CdS photocatalyst is a prospective candidate for solar hydrogen generation from water-splitting.

In this report, PtNi_x alloy NPs coupled with a CdS photocatalyst for photocatalytic hydrogen generation under visible light have been explored.