Main Chain Length Research Articles

Uncovering the mechanism controlling strength and microstructural evolution of belite-rich cement-based materials at different temperatures

The effect of temperature on the properties of belite-rich cement-based (BRC) materials is different from that on Portland cement (PC). This study provides a comprehensive understanding of the mechanisms controlling strength and microstructural evolution at different temperatures, especially C-S-H characteristics, of BRC materials. Phase assemblage, pore structure, and microscopic morphology of BRC paste, as well as phase composition and structure, chemically bound water, and bulk density of C-S-H were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TGA), 1H and 29Si nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The strength of BRC mortar increased with temperature is because of the higher hydration degree of β-C2S, lower porosity (except for 60 °C), and increased main chain length (MCL) of C-S-H. Hydration degrees of 28-d β-C2S and C3S increased by 202.5% and 16.3%, respectively, while MCL increased by 39.4% from 5 to 60 °C. The higher temperature sensitivity of β-C2S is due to its activation energy being 12.4% greater than that of C3S in BRC paste. Additionally, gel porosity decreased with temperature due to decreased bound water content and increased 16.7% bulk density of C-S-H from 5 to 60 °C. Finally, the CaO/SiO2 was inversely proportional to MCL and bulk density, but positively to H2O/SiO2. The findings deepen the mechanistic understandings of the hydration kinetics and microstructural evolution for temperature-affected BRC material.

Influence of Polymer Side Chain Size and Backbone Length on the Self-Assembly of Supramolecular Polymer Bottlebrushes.

Hydrogen bonds are a versatile tool for creating fibrous, bottlebrush-like assemblies of polymeric building blocks. However, a delicate balance of forces exists between the steric repulsion of the polymer chains and these directed supramolecular forces. In this work we have systematically investigated the influence of structural parameters of the attached polymers on the assembly behaviour of benzene trisurea (BTU) and benzene tris(phenylalanine) (BTP) conjugates in water. Polymers with increasing main chain lengths and different side chain sizes were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of hydroxyethyl acrylate (HEA), tri(ethylene glycol) methyl ether acrylate (TEGA) and oligo(ethylene glycol) methyl ether acrylate (OEGA). The resulting structures were analyzed using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Both BTU and BTP formed fibres with PHEA attached, but a transition to spherical morphologies was observed at degrees of polymerisation (DP) of 70 and above. Overall, the main chain length appeared to be a dominating factor in inducing morphology transitions. Increasing the side chain size generally had a similar effect but mainly impeded any aggregation as is the case of POEGA. Interestingly, BTP conjugates still formed fibres, suggesting that the stronger intermolecular interactions can compensate partially for the steric repulsion.

Effect of graphene oxide on the hydration process and macro-mechanical properties of cement

The impact of graphene oxide (GO) on the hydration process, calcium silicate hydrate (CSH) gels structure, and macro-mechanical properties were systematically researched by combinatorial techniques. Findings from 29Si MAS-NMR and nitrogen adsorption (BET) revealed that the effect of GO on the hydration degree of the cement paste, and the main chain length (MCL) is more pronounced at advanced ages (from 28 days), due to its act as a nucleation site. Moreover, the results of Raman spectroscopy tests showed that GO has a strong interaction with the cement matrix. Due to the increase in the degree of hydration, the lengthening of the chain length (MCL), and the formation of strong bonds, both compressive and flexural strength tests also improved. Therefore, the effect of GO as a nucleation site has a positive effect on the cement paste nano-properties at advanced ages.

Influence of Graphene Oxide on Mechanical Properties and Durability of Cement Mortar.

The effect of graphene oxide (GO) on the mechanical strengths and durability of cement composites was researched by preparing GO-modified cement mortars. Thermogravimetric analysis (TGA) and nuclear magnetic resonance (29Si MAS-NMR) were performed on the cement paste to evaluate the influence of GO on the hydration process and chain structure of calcium-silicate-hydrate (C-S-H) gels. TGA revealed that the high GO dosage increased the content of C-S-H by 5.46% compared with the control at 28 days. Similarly, 29Si-NMR improved the hydration degree and main chain length (MCL) in GO-modified samples at 28 days. The GO led to increases of 2.54% and 7.01% in the hydration degree and MCL, respectively, compared with the control at 28 days. These findings underscore the multifaceted role of GO in improving the mechanical properties and durability of cement composites. Mechanical strength tests, such as compressive and flexural tests, were conducted on cement mortars. The optimal dosage of GO increased the compressive strength by 9.02% after 28 days. Furthermore, the flexural strength of cement mortars with the combination of GO and superplasticizer (SP) after 28 days increased by 21.86%, compared with reference mortar. The impact of GO proved to be more pronounced and beneficial in the durability tests, suggesting that GO can enhance the microstructure through hydration products to create a dense and interconnected microstructure.

Syntheses, properties and mechanistic studies of 8-aminooctanoic acid-modified polyaspartic acid polymers for calcium scale inhibition

The anti-scaling agent poly(aspartic acidic) (PASP) polymers had became a hot research topic based on its peptide bond structures with degradation characteristics. However, it was difficult to control the chain length distributions of the synthesized products, which led to difficulty in determining the scaling mechanism for the peptide bond products. In this paper, PASP/8AC graft modified polymers with a narrow dispersity (Ð) and controllable chain lengths was synthesized via a simple two-step method, and 8-aminooctanoic acid (8AC) was introduced into the peptide bond structure of PASP with main chain lengths of 12. The effects of the side-chain lengths and functional groups on the scale inhibition mechanism were determined. The static scale inhibition results showed that the grafted products had significantly improved scale inhibition efficiencies at low concentrations and good temperature and time stabilities. The inhibition rate for CaSO4 scaling was increased by almost 60 % at a concentration of 3 mg/L. At 5 mg/L, the inhibition rate for CaCO3 scaling was increased by 36 %. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) showed that side chain length extensions improved calcium ion chelation by the PASP and increased calcium scale lattice distortion. This study verified the effects of side chains on PASP scale inhibition and its mechanism. This is of great importance in designing new green water treatment agents.

Oligofluorene main-chain length-dependence on one-pot extraction of chirality-selective semiconducting single-walled carbon nanotubes

Abstract Polyfluorene and their derivatives are noted for being fascinating dispersants for single-walled carbon nanotubes (SWCNTs) because they selectively solubilize semiconducting (sem-) SWCNTs. However, the selective extraction mechanism of this unique behavior has not yet been fully clarified. In this paper, we describe a unique SWCNT solubilization behavior using 12 fluorene oligomers with different main-chain lengths (FOn, n = 2∼30), in which n is the number of fluorene-repeating unit. Sonication of SWCNTs using these oligofluorenes in toluene was found to solubilize SWCNTs when the main-chain length was longer than n = 12. Raman spectra revealed that selective sem-SWCNT extraction occurred when using the FOn with n ≥ 18, while, when using FOn (n = 12, 15), both sem- and metallic (met-) SWCNTs were solubilized. The (n,m)chiralites of the extracted SWCNTs using the fluorene oligomers differed from those using a homopolymer, poly(9,9′-di-n-octylfluorene) (PFO); that is, PFO extracted (9,7)SWCNTs well, while only FO30 slightly dissolved the (9,7)tubes, and when using other FOn (n = 12∼27), no (9,7)tubes were solubilized. The present study demonstrated the importance of the main-chain length of the oligofluorenes on selected chirality extraction of sem-SWCNTs, which is useful for designing fluorene-based compounds with selective extraction of sem-SWCNTs with a specific (n,m) chirality.

Characterization and Antioxidant Activity of the Polysaccharide Hydrolysate from Lactobacillus plantarum LPC-1 and Their Effect on Spinach (Spinach oleracea L.) Growth.

This study presents a comparison between two hydrolysis systems (MnO2/H2O2 and ascorbic acid (VC)/H2O2) for the depolymerization of exopolysaccharide (EPS) from Lactobacillus plantarum LPC-1. Response surface methodology (RSM) was used to optimize these two degradation systems, resulting in two H2O2-free degradation products, MEPS (MnO2/H2O2-treated EPS) and VEPS (VC/H2O2-treated EPS), where H2O2 residues in the final products and their antioxidant activity were considered vital points. The relationship between the structural variations of two degraded polysaccharides and their antioxidant activity was characterized. Physicochemical tests showed that H2O2 had a notable impact on determining the total and reducing sugars in the polysaccharides, and both degradation systems efficiently eliminated this effect. After optimization, the average molecular weight of EPS was reduced from 265.75kDa to 135.41kDa (MEPS) and 113.11kDa (VEPS), improving its antioxidant properties. Characterization results showed that the two hydrolysis products had similar major functional groups and monosaccharide composition as EPS. The crystal structure, main chain length, and branched chain number were crucial factors affecting the biological activity of polysaccharides. In pot testing, two degraded polysaccharides improved spinach quality more than EPS due to their lower molecular weights, suggesting the advantages of low-molecular-weight polysaccharides. In summary, these two degradation techniques offer valuable insights for further expanding the utilization of microbial resources.

Dual-modal imaging-guided agent based on NIR-II aggregation-induced emission luminogens with balanced phototheranostic performance.

Phototherapy has garnered considerable interest for its potential to revolutionize conventional cancer treatment. Organic materials with near-infrared II (NIR-II, 1000-1700 nm) fluorescence and photothermal effects are key for precise tumor diagnosis and treatment, yet optimizing their output for higher resolution and reduced photodamage remains a challenge. Herein, a multifunctional NIR-II photosensitizer (LSC) has been developed using the aggregation-induced emission (AIE) technology. The utilization of thieno[3,2-b]thiophene as an electron-rich and bulky donor/acceptor bridge has allowed for the elongation of conjugation length and distortion of the AIE main chain. This strategic modification effectively enhances the electron push-pull effect, endowing the LSC with a Stokes shift of over 400 nm and AIE characteristics. We have successfully built-up stable nanoparticles called FA-LSC NPs using a nano-precipitation method. These nanoparticles exhibit high NIR-II fluorescent brightness (ε × QY = 1064 M-1 cm-1) and photothermal conversion efficiency (41%). Furthermore, the biocompatible FA-LSC NPs demonstrate effective tumor accumulation and exceptional photothermal therapeutic efficacy both in vitro and in vivo. These nanoparticles were applied to fluorescence-photothermal dual-mode imaging-guided photothermal ablation in a HeLa tumor xenograft mouse model, resulting in favorable photothermal clearance outcomes.

Interaction of Polycarboxylate-Based Water-Reducing Admixture Molecular Structure with Fly Ash Substituted Cementitious Systems: Marsh-Funnel and Mini-Slump Performance

Polycarboxylate ether-based high-range water-reducing admixtures (PCE) play a crucial role in enhancing the workability of fly ash-substituted cementitious systems. Alterations to the chemical structures of both the main and side chains can particularly enhance the electrostatic repulsion and steric hindrance effects of PCEs. As a consequence, this results in improved performance within cementitious systems containing fly ash substitutions. This study investigated the compatibility of the altered molecular structure of polycarboxylate ether (PCE) with cementitious systems containing fly ash substitutions. To achieve this, five polymers were synthesized, varying the side chain length, molecular weight, main chain length, and main chain lengths of PCEs while maintaining other properties constant. Cement pastes were then prepared using the synthesized PCEs with fly ash replacements at three different rates. Marsh-funnel flow time and mini-slump values were measured in the prepared mixtures. The study revealed that in mixtures without fly ash, PCEs with a long main chain (40k) and short side chain length (1000 g/mole) exhibited the lowest Marsh-funnel flow and mini-slump performance among PCEs with diverse molecular structures. For the other PCEs in these mixtures, the change in molecular structure did not significantly affect their performance. However, as the fly ash replacement rate increased, PCE having a medium main chain (21k) and side chain (2400 g/mole) length outperformed other PCEs in terms of Marsh-funnel flow and mini-slump performance.

Ignition and combustion characteristics of ultrasonically levitated single droplets of nano‑boron/oxygenated fuel slurries

Addition of boron nanoparticles to liquid oxygenated fuel lead to the formation of nano‑boron slurries with high energy density. This study focuses on ignition and combustion characteristics of single droplets of nano‑boron slurries during ultrasonic levitation and effects of different oxygenated fuels on ignition and combustion characteristics of single droplets. In this study, nano‑boron slurries of boron/ethanol, boron/1-hexanol, boron/dimethyl carbonate, boron/triacetate, boron/propylene glycol monomethyl ether and boron/tripropylene glycol monomethyl ether with 10 wt% solid loading were prepared. Moreover, an ultrasonic levitation system was designed and established, which was used for igniting single droplets in ultrasonic levitation state. The results indicate that ignition and combustion process of levitated single droplets of nano‑boron slurries can be divided into three stages: a stable endothermic stage, an oscillation and micro-explosion stage and an aerosol combustion stage. Smoke was observed before aerosol combustion stage of boron/triacetate and boron/tripropylene glycol monomethyl ether, whereas flame jets were observed during combustion of other fuels. Furthermore, emission spectra of BO2 with high intensity could be observed during aerosol combustion stage. A shorter main chain length of oxygenated fuel facilitates ignition of nano-B slurries. Boron/propylene glycol monomethyl ether exhibited shortest ignition delay, which was approximately 13.8% shorter than that of boron/tripropylene glycol monomethyl ether with the longest ignition delay. Furthermore, a longer main chain length of oxygenated fuel facilitates combustion of nano-B slurries. Nano‑boron slurries with longer main chains (boron/1-hexanol, boron/triacetate and boron/tripropylene glycol monomethyl ether) exhibited brighter and larger flames, regardless of whether they were alcohols, esters or ethers, and quenched zones with bright and clearly delineated edges were observed. Intensity integral and maximum intensity of emission of these three fuels were also higher than those of other fuels. Boron/triacetate demonstrated the best combustion performance, with maximum intensity and intensity integral at 547.8 nm being 3.88 times and 1.9 times those of boron/ethanol, respectively, which has the worst combustion performance. This study provides a theoretical basis and experimental reference for the application of oxygenated-fuel-based nano‑boron slurries and their base liquid fuel selection.

Single and Multi Effects of Polycarboxylate Main and Side Chain Lengths on Setting Time, Viscosity, and Yield Stress of Cementitious Mixtures Containing Fly Ash

Single and Multi Effects of Polycarboxylate Main and Side Chain Lengths on Setting Time, Viscosity, and Yield Stress of Cementitious Mixtures Containing Fly Ash

Effect of Ca/Si ratio on the characteristics of alkali-activated ultra-high performance concrete (A-UHPC): From hydration kinetics to microscopic structure development

Ultra-high performance concrete (UHPC) produces higher carbon emissions than ordinary concrete. To achieve sustainable development of UHPC, this article explores the use of alkali-activated technology to create a low-carbon UHPC, in which the effect of different Ca/Si ratios (from 0.7 to 1.5) on the performance of alkali-activated UHPC (A-UHPC) is detailed analyzed. The obtained results show that with an increase of Ca/Si ratio, the properties of A-UHPC firstly increases and then gradually decreases. When the Ca/Si ratio is 1.2, the performance of the developed A-UHPC can be optimized, such as the 28-day compressive strength is 121.3 MPa, the relative content of C-(A)-S-H gel is 99.38%, the main chain length is 14.27, and the porosity is 4.28%. In general, this study can provide guidance for the formulation design and production of a new low carbon construction material in the future.

Flowability, Strength, and Water Absorption of Mortars Containing Fly Ash and WRA Having Varying Main Chain Lengths

Within the scope of this study, the effect of the main chain length of polycarboxylate-based water-reducing admixture (WRA) on the fresh properties, compressive strength, and water absorption of mortar mixtures containing fly ash was investigated. Three WRAs with the same structure but different main chain lengths and resultantly different molecular weights were used. Effects of the admixtures on the properties of cementitious systems with 0, 15, 30, and 45 wt.% fly ash were investigated. Irrespective of the fly ash substitution level, the fluidity of the mixtures was improved when the admixture main chain length was increased up to a certain level. Possibly beyond a certain main chain length, the entanglement of too large main chains of the polymer to each other reduced the electrostatic effect of the polymer. Thus, the flow properties of the mixtures were affected negatively. The side chain molecular weight (length) of the admixtures was fixed (2,400 g/mol). However, the molecular weight of the admixtures was adjusted as 24, 48, and 71 kg/mol by changing the main chain length of the polymer. The admixture with 48 kg/mol molecular weight showed the best performance in terms of mortar fresh properties. However, the change in the length of the main chain of admixture did not have a considerable effect on the compressive strength and water absorption of the mortar. In addition, apart from the WRA admixture property, time-dependent flow and permeation properties of the mixtures were affected negatively by the increase in fly ash substitution level.

Combined interaction of PCE chains lengths, C3A and water content in cementitious systems

In this study, the interaction of polycarboxylate ether-based water reducing admixture (PCE) molecule chain length variation with cements having different C3A contents and water content was investigated. For this purpose, a total of seven PCEs were synthesized following three different approaches, i.e., the main chain length changed at a constant side chain length, the side chain lengths varied at a constant main chain length, and both main and side chain lengths changed at a similar molecular weight. Paste and mortar mixtures were prepared with the PCEs and CEM I-42.5R type cements containing three different C3A contents (3.60%, 6.82%, 9.05%). The effects of above mentioned variables on the rheological properties and compressive strength of the paste/mortar mixtures were determined. The interaction of PCEs with cements having different C3A contents was investigated in detail by performing total organic carbon (TOC) and dynamic light scattering (DLS) analyses. The increase in the main and side chain lengths of PCE molecule up to a certain point positively affected the fresh properties of cementitious systems. Beyond a certain point (i.e. when main chain length is higher than 21 k), the increase in main chain length reduced the hydrodynamic diameter of PCE molecules (up to 32%), and decreased the dispersion performance of the admixture. The increment in side chain length (i.e. when side chain length is higher than 2100 g/mole), on the other hand, adversely affected the rheological properties, especially with high C3A cements.

Modification mechanism of cellulose nanocrystals in cement

To investigate the modification mechanism of cellulose nanocrystals (CNCs) in cement, this paper simplifies the complex cement system to pure tricalcium silicate (C3S) mineral components. The hydration characteristics of C3S mixed with different amounts of CNCs and the chemical environment of the hydration products were explored. CNCs have a similar effect to superplasticizers in reducing the hydration exothermic. The addition of CNCs promotes the growth of Ca(OH)2 on the (001) crystal plane and increases the grain size of Ca(OH)2. CNCs provide additional nucleation sites, which reduces the degree of polymerization and shortens the main chain length of the generated C-S-H. The separate-hydration test confirmed that the CNCs successively adsorbed calcium ions and silicate ions, which promoted the formation of C-S-H gel coating on the surface of CNCs. The elucidation of the modification mechanism of CNCs provides theoretical guidance for its application in cement.

Hardened oil well cement paste modified with TiO2@SiO2 nanoparticles: Physical and chemical properties

Core/shell TiO2@SiO2 nanoparticles (nTS) were added to cement class G, and their physical and chemical properties were investigated with and without polycarboxylate (PC). The hardened cement pastes were analyzed through gas pycnometry, Brunauer-Emmett-Teller (BET), field emission scanning electron microscopy (FE-SEM), and compressive strength. For hydration characterization, portlandite quantification, and CSH analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and 29Si magic-angle spinning nuclear magnetic resonance (29Si MAS NMR) were performed. The nTS demonstrated a self-dispersive behavior and increased the specific mass and matrix compacity. Moreover, the total pore volume decreased, and the compressive strength increased. The nTS improved the hydration degree, reduced portlandite content, and increased CSH main chain length.

Glucose‐derived superabsorbent hydrogel materials based on mechanically‐interlocked slide‐ring and triblock copolymer topologies

AbstractA series of glucose‐based degradable superabsorbent hydrogels with potential to tackle issues associated with sustainability, flooding, and drought has been designed and fabricated. These hydrophilic networks were constructed through integrating glucose as a primary building block –into cyclic oligomers and block polymers, which were combined into mechanically‐interlocked slide‐ring crosslinked materials. Crosslinking of slide ring α‐cyclodextrin/poly(ethylene glycol)‐type polyrotaxanes with acid‐functionalized ABA triblock copolymers comprised of mercaptopropionic acid‐functionalized poly(glucose carbonate (ethyl propargyl carbonate))‐b‐poly(ethylene glycol)‐b‐mercaptopropionic acid‐functionalized poly(glucose carbonate (ethyl propargyl carbonate)), afforded degradable superabsorbent hydrogels through establishment of chemically‐labile ester linkages, in addition to glycosidic and carbonate groups of the polymer precursors. With an emphasis on development of fundamental synthetic design strategies to achieve high‐performance superabsorbent hydrogels that could behave as robust materials, which are derived from natural components and exhibit hydrolytic degradability, effort went into optimization of the composition, structure, and topology leading to water uptake capacities >30× by mass. Investigations of composition‐structure‐topology‐morphology effects on properties as a function of variations of PEG main chain length, degree of α‐cyclodextrin coverage, and concentration of pre‐gel solution, indicated that the slide‐ring polymer and triblock copolymer networks feature high water uptake, tunable mechanical properties, and sustainability with construction from renewable natural products and in‐built degradability.

Tuning Cocrystallization, Microphase Separation, and Optical Property of All-Conjugated Triblock Copolymers by Molecular Engineering

Compared to the widely studied classical coil–coil block copolymers (BCPs) with well-established phase diagram and recent advances achieved in all-conjugated diblock copolymers, the investigation into the phase behavior of all-conjugated triblock copolymers is rather limited. In this work, we prepare a series of triblock copolymers containing various poly(3-alkylthiophene)s (P3ATs), interrogate their phase transition between cocrystallization and microphase separation adjusted by a set of molecular parameters (alkyl side chains, block ratios, and block sequences), and correlate these different structures strongly to their optical properties. Poly(3-butylthiophene) (P3BT), poly(3-hexylthiophene) (P3HT), poly(3-(2′-ethyl)hexylthiophene) (P3EHT), poly(3-octylthiophene) (P3OT), and poly(3-dodecylthiophene) (P3DDT) are selected, in which P3EHT has a much less crystalline capability than the other four components. Interestingly, when the P3EHT block is situated in the BCP middle position (P3BT-b-P3EHT-b-P3AT), a smaller side chain difference between the two outer P3BT and P3AT blocks and the P3EHT with a shorter main chain length favor cocrystallization in the triblock copolymers over microphase separation. Moreover, when changing the block sequence to place the P3EHT block at two terminals (P3OT-b-P3BT-b-P3EHT and P3BT-b-P3OT-b-P3EHT), they display stronger cocrystalline capability than P3BT-b-P3EHT-b-P3OT with the P3EHT in the central. At last, these various crystalline structures are related to their optical properties. This work demonstrates the tuning of the phase transition between cocrystalline and microphase-separated structures in conjugated triblock copolymers via molecular engineering, which correlate closely to their optical properties and may underpin their applications in optoelectronic devices.

An efficient approach for sustainable fly ash geopolymer by coupled activation of wet-milling mechanical force and calcium hydroxide

Geopolymer is a potential substitute for high-emission cement production. Therefore, the use of fly ash with high emission as a geopolymer is an environmentally friendly and inexpensive direction. However, fly ash (FA) is a high amorphous precursor with low pozzolanic reactivity making it difficult to be activated by calcium hydroxide. In this study, the coupling of wet-milling mechanical force and calcium hydroxide was used to prepare high-performance alkaline calcium activated geopolymers. Two kinds of FA slurries with different particle sizes of D50 = 2.96 μm and 14.2 μm were prepared, activated by calcium hydroxide (CH) with the content of 4%, 11% and 19%. Results indicated that the increase of calcium hydroxide content was beneficial to developing strength, effectively improving the chloride resistance, compacting the microstructure, but increasing the autogenous shrinkage of the geopolymers. These improvements are especially apparent in the wet-milled fly ash geopolymers (WFA) due to the pre-depolymerization implemented by wet-milled mechanical forces, and coupled with the activation effect of CH to improve the depolymerization efficiency and condensation reaction. The compressive strength of WF-CH-19 was three times higher than that of FA-CH-19, reaching 29.3 MPa at 28 d, and the compressive strength growth of WF-CH-11 even reached 591.67% at 1 d. Meanwhile, the main chain length (MCL) and Al/Si of calcium silicate hydrates were clearly improved, and pore structure was significantly refined with capillary pore increased from 29.79% to 89.23%. In addition, FA and WFA geopolymers have significant advantages over Portland cement in the environmental impact indicators such as E-energy and E-CO2.

Photoluminescence mechanisms of red-emissive carbon dots derived from non-conjugated molecules

Red-emissive carbon dots (R-CDs) have been widely studied because of their potential application in tissue imaging and optoelectronic devices. At present, most R-CDs are synthesized by using aromatic precursors, but the synthesis of R-CDs from non-aromatic precursors is challenging, and the emission mechanism remains unclear. Herein, different R-CDs were rationally synthesized using citric acid (CA), a prototype non-aromatic precursor, with the assistance of ammonia. Their structural evolution and optical mechanism were investigated. The addition of NH3·H2O played a key role in the synthesis of CA-based R-CDs, which shifted the emission wavelength of CA-based CDs from 423 to 667 nm. Mass spectrometry (MS) analysis indicated that the amino groups served as N dopants and promoted the formation of localized conjugated domains through an intermolecular amide ring, thereby inducing a significant emission redshift. The red-emissive mechanism of CDs was further confirmed by control experiments using other CA-like molecules (e.g., aconitic acid, tartaric acid, aspartic acid, malic acid, and maleic acid) as precursors. MS, nuclear magnetic resonance characterization, and computational modeling revealed that the main carbon chain length of CA-like precursors tailored the cyclization mode, leading to hexatomic, pentatomic, unstable three/four-membered ring systems or cyclization failure. Among these systems, the hexatomic ring led to the largest emission redshift (244 nm, known for CA-based CDs). This work determined the origin of red emission in CA-based CDs, which would guide research on the controlled synthesis of R-CDs from other non-aromatic precursors.