{"id":"df7fec4a-a710-475c-ba5b-5fc965fbe4fb","arxiv_id":"2505.10886","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Varying the HCl dose during lignin acidolysis tunes the surface chemistry and luminescence of the resulting carbon quantum dots, with a small HCl dose producing a distinct green emission band.","lead":"Spruce lignin was converted into carbon quantum dots using different amounts of hydrochloric acid before a hydrothermal step. The acid dose changed the dots' surface chemistry, and a small dose produced a distinct green emission while the others glowed blue.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Green emission in the 20 μl HCl sample is assigned to surface states without controls excluding molecular fluorophores or absorption evidence for a reduced HOMO-LUMO gap.","rationale":"The reader's weakest assumption identifies a real gap in the paper: the surface-state origin of the distinct green emission is inferred from correlation (XPS, NMR, zeta potential) and schematic models, not from direct evidence that rules out alternative emitters. My sharper formulation focuses on the molecular-fluorophore confound and the missing absorption/quantum-yield data, both of which are standard checks in carbon-dot research. The paper gives consistent multi-technique characterisation and the synthesis is reproducible in principle, so the weakness is addressable rather than fatal. A conditional verdict is appropriate: the mechanistic conclusions should be presented as tentative until control experiments exclude molecular fluorophores and optical data substantiate the claimed gap reduction. I do not see an internal inconsistency or a fundamental error; the concern is about the strength of evidence for the central claim, matching the reader's assessment. Therefore the verdict should remain CONDITIONAL (UNCHANGED).","tokens_in":841,"tokens_out":750,"duration_ms":48201,"concrete_test":"Run control hydrothermal syntheses under the 20 μl HCl protocol with (i) m-aminophenylboronic acid alone and (ii) lignin alone (no dopant), each purified by the same dialysis, and compare emission spectra at matched optical density. If the ~503 nm green band appears in either control, molecular fluorophores are the likely origin rather than LG-CQD surface states; if it is absent, the surface-state assignment is strengthened.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central mechanistic claim is that the 503 nm green emission of N,B CQD 20 μl HCl arises from surface-state radiative recombination with a reduced HOMO-LUMO gap (Luminescence section; Conclusions). The supporting evidence is a schematic surface model (Fig. 4), an energy diagram (Fig. 7a), excitation-dependent emission, and a single-exponential lifetime. This assignment is underdetermined. (1) The paper itself lists molecular fluorescence as a possible mechanism but reports no control syntheses: no hydrothermal run with m-aminophenylboronic acid alone, or with lignin alone, under the same 20 μl HCl pretreatment and dialysis. Small molecular fluorophores could survive 500–1000 Da dialysis and dominate the 503 nm band. (2) No absorption spectra or absolute quantum yields are reported; a genuine reduction in the HOMO-LUMO gap should appear as a lower-energy absorption onset or a corresponding excitation feature. Without this, the emission could reflect excited-state charge transfer, aggregation, or inner-filter effects. (3) The 20 μl sample differs from the others in particle size (core 1.7 nm vs ~10 nm) and in agglomerate size distribution, and in concentration (0.19 vs 0.03 mg/ml); because emission spectra are shown only normalized, the relative intensity of the 503 nm band is not a clean readout of surface chemistry. The 'increased number of bonds' observed by HSQC for the 20 μl sample is explicitly acknowledged as small and less reliable, yet it is used to support the distinct core structure.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a systematic study of lignin-derived carbon quantum dots (LG-CQDs) synthesized from spruce kraft lignin by a two-step acidolysis/hydrothermal route using m-aminophenylboronic acid as a dopant and three different HCl pretreatments (0, 20, and 1000 µL). The authors characterize the resulting materials by TEM, DLS, FTIR, Raman, XPS, 2D-HSQC NMR, zeta potential, and photoluminescence spectroscopy. They find that HCl dose correlates with increased surface incorporation of N, B, and Cl, with higher graphitic N content and more ordered sp2 domains, and they propose a formation mechanism involving de-etherification of lignin and subsequent condensation of doped monomeric units. A central claim is that the 20 µL HCl sample exhibits distinctly enhanced green emission at 503 nm, attributed to surface-state radiative recombination enabled by a reduced HOMO-LUMO gap caused by altered surface chemistry. The paper concludes that the acidolysis step is crucial for tuning the luminescence of LG-CQDs.","tokens_in":14567,"tokens_out":3405,"duration_ms":33913,"significance":"If the central mechanistic claim were fully supported, the paper would be a useful contribution to the rational design of LG-CQDs with tailored emission, and the multi-technique structural characterization (XPS, HSQC, Raman, TEM/DLS) of the three samples is in itself valuable. The authors are commendably explicit about the small and less reliable NMR bond-count differences for the 20 µL sample and about the complexity of assigning carbon-dot luminescence mechanisms. However, the headline conclusion—that surface functional groups directly control the green emission via a reduced HOMO-LUMO gap—is currently underdetermined by the data presented. The paper would benefit from additional control experiments and optical characterization to make the mechanistic claim load-bearing rather than speculative.","major_comments":[{"comment":"The assignment of the 503 nm green emission in the 20 µL HCl sample to surface-state recombination with a reduced HOMO-LUMO gap is not sufficiently supported because the authors do not rule out molecular fluorophores. The manuscript itself lists molecular fluorescence as one of the general luminescence mechanisms in CQDs, yet no control syntheses are reported: there is no hydrothermal run with m-aminophenylboronic acid alone, or with lignin alone, under the same HCl pretreatment and dialysis conditions. Given that the dialysis cutoff is only 500-1000 Da, small molecular fluorophores produced during acidolysis or from the dopant could survive and dominate the 503 nm band. Without these controls, the surface-state interpretation is one of several equally plausible explanations.","section":"Luminescence of CQDs (Fig. 7; Conclusions)"},{"comment":"No absorption spectra or absolute quantum yields are reported, although these are standard and decisive for testing a reduced HOMO-LUMO gap. A genuine reduction in the energy gap should manifest as a lower-energy absorption onset or a corresponding excitation feature; the excitation spectra shown in Fig. 7e contain bands only up to about 370-400 nm, and the 503 nm emission is excited at 400-480 nm. The absence of absorption data means the green band could equally arise from excited-state charge transfer, aggregation, or inner-filter effects, none of which require a reduced fundamental gap.","section":"Luminescence of CQDs (Fig. 7); Characterization methods"},{"comment":"The comparison of emission spectra across the three samples is confounded by differences in particle size, agglomeration, and concentration. The 20 µL HCl sample has a mean core diameter of 1.7 ± 0.3 nm versus approximately 10 nm for the no-HCl sample, different agglomerate size distributions (118.6 ± 39.5 nm vs 290 ± 54 nm), and a six-fold higher initial concentration (0.19 vs 0.03 mg/mL). Emission spectra are shown only in normalized form (Fig. 7e-f), so the apparent increase in relative intensity of the 503 nm band does not cleanly isolate surface chemistry from size, aggregation, or concentration effects. Concentration-matched or size-fractionated samples, or reporting absolute emission intensities and quantum yields, are needed to substantiate the claim.","section":"Results: Structure and surface chemistry (Fig. 1); Luminescence (Fig. 7)"},{"comment":"The NMR evidence for a distinct structure in the 20 µL HCl sample is internally flagged as weak, yet it is later used as a load-bearing part of the mechanistic argument. The text states that after acidolysis the number of propanoid bonds is only 0-8 per 1000 aromatic units, that the relative proportions are 'less reliable' at such low numbers, and that the 'slight increase' for 20 µL HCl 'is small in comparison to the parent lignin.' Nevertheless, the Luminescence section and Conclusions invoke the 'increased number of bonds after acidolysis' for this sample to explain its unusual emission. This inconsistency should be reconciled, either by stronger quantitative NMR evidence or by removing this element from the mechanistic claim.","section":"NMR study and mechanism approach (Fig. 5); Luminescence and Conclusions"}],"minor_comments":[{"comment":"The caption of Figure 7 contains labeling errors: it lists 'e' twice (excitation spectra and emission spectra) and then uses 'f' and 'h' for decay curves, which is inconsistent and confusing. Please renumber the panels and ensure each panel is referenced correctly in the text.","section":"Luminescence of CQDs (Fig. 7 caption)"},{"comment":"The manuscript reports XPS compositions, zeta potentials, DLS sizes, and luminescence lifetimes as exact values without replicates, standard deviations, or error bars. For quantitative claims such as the N-graphitic/N-amino ratio trend in Table 1, at least duplicate or triplicate measurements with uncertainties are expected.","section":"Throughout"},{"comment":"The caption of Figure 5b refers to 'pine kraft lignin' while the text and title consistently discuss spruce kraft lignin; this inconsistency should be corrected.","section":"NMR study and mechanism approach (Fig. 5b)"},{"comment":"The three syntheses are described as 'three different approaches' in the abstract, but the only variable is the HCl volume added during the pretreatment; 'approaches' overstates the distinction. Consider rephrasing to 'three different HCl dosages.'","section":"Materials and methods (Synthesis)"},{"comment":"The C 1s fit is described as having seven components, with binding energies listed as 284.5, 284.9, 286.2, 288.9, 289.8, 285.7, and 284.0 eV, but the order does not match the assignments given (C=C, C-C, C=O, C-O, C-N, C-B). Please present the energies and assignments in a consistent order to avoid ambiguity.","section":"XPS analysis (Fig. 3)"},{"comment":"References [9] and [13] are the same publication (Zhu et al., Appl Surf Sci 662, 2024) and should be merged or renumbered.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The structural characterization and the observed correlation between HCl dose and surface composition are solid and publishable in principle. My main concern is that the central luminescence-mechanism claim (green emission from surface states with a reduced HOMO-LUMO gap in the 20 µL HCl sample) is not yet proven: the missing control syntheses, absent absorption/QY data, and confounded sample comparison leave the interpretation underdetermined. This is fixable with additional experiments, so I recommend major revision rather than rejection. The paper would also benefit from a more cautious framing: the data support a phenomenological correlation between synthesis conditions and emission color, whereas the mechanistic attribution requires the additional evidence."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a solid characterization study with one genuinely useful knob—HCl dose in the acidolysis step—and one overreach: the green emission mechanism attributed to surface states is not actually shown.\n\nWhat's new: prior work used aminophenylboronic acid with lignin, but systematically varying HCl (none, 20 µl, 1000 µl) and correlating dose with surface chemistry is a real addition. The XPS N/B/Cl trends, zeta potential shifts, and decreasing Raman ID/IG all line up consistently. The 2D HSQC NMR tracking of ether bond cleavage is a nice piece of evidence, and the authors are appropriately cautious about the unreliable low bond counts at 20 µl. The surface models in Figure 4 are consistent with the data.\n\nSoft spots: the central mechanism is underdetermined. The paper lists molecular fluorescence as a possible origin but runs no control syntheses (no lignin-only or dopant-only hydrothermal runs), and 500–1000 Da dialysis can pass small fluorophores. There are no absorption spectra or absolute quantum yields, so the claimed reduced HOMO-LUMO gap has no direct spectroscopic support. The 20 µl sample also differs from the others in particle size (1.7 vs ~10 nm core) and concentration (0.19 vs 0.03 mg/ml), and emission spectra are normalized, so the green band's prominence is not a clean readout of surface chemistry. Also, the statement that HCl pretreatment is 'essential' sits oddly next to their own HCl-free N,B CQD sample, which formed fine; 'essential' should be softened to something like 'modifies particle formation.'\n\nThese are fixable with additional experiments, not fundamental contradictions. The characterization data are worth having, and the HCl-dose correlation is a useful synthesis guide. It deserves a serious referee, but the referee should push for control syntheses and absorption data before the mechanism is accepted.","headline":"A competent multi-technique study showing HCl dose tunes the surface chemistry of lignin carbon dots, but the green-emission mechanism is asserted rather than demonstrated.","tokens_in":14996,"tokens_out":2645,"would_cite":true,"duration_ms":27078,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper argues that the amount of hydrochloric acid used in the acidolysis step controls which functional groups end up on lignin carbon quantum dots, and that a specific small acid dose creates a surface state that emits green light…","keywords":["lignin carbon quantum dots","spruce biomass","acidolysis","surface chemistry","photoluminescence","N,B co-doping","green emission","hydrothermal synthesis"],"falsifier":"Fractionate the 20 μl HCl sample by size-exclusion chromatography or stepwise centrifugation and measure the emission of each fraction: if the 503 nm band survives in the smallest well-dispersed particles after removal of low-molecular-weight species, the surface-state explanation is supported; if it disappears or shifts with aggregation state, the surface-state assignment fails.","tokens_in":14001,"feed_emoji":"🌲","tokens_out":5387,"duration_ms":55969,"temperature":0.7,"pith_summary":"The paper studies how lignin from spruce biomass is converted into carbon quantum dots by acidolysis followed by hydrothermal reaction, using m-aminophenylboronic acid as a nitrogen and boron dopant and different amounts of hydrochloric acid. It tries to establish that the acidolysis step is not just a preprocessing stage: the HCl amount determines which fragments of lignin survive, which dopant groups attach, and therefore what color the dots emit. The central result is that a small HCl dose (20 μl) produces dots with a distinct green emission peak at 503 nm, while no acid or a large dose (1000 μl) gives similar blue-dominated emission. This matters because it offers a simple synthesis knob for tuning luminescence in a sustainable biomass-derived material.","feed_headline":"Acid dose tunes green glow of lignin quantum dots","feed_subtitle":"Twenty microliters of HCl shifts spruce-lignin carbon dots to a 503 nm green emission; more acid does not.","key_machinery":"The load-bearing machinery is the two-step acidolysis-and-hydrothermal synthesis combined with the proposed surface-state luminescence model. Acidolysis with HCl and m-aminophenylboronic acid de-etherifies the lignin network and grafts nitrogen and boron onto the remaining aromatic units; the hydrothermal step then condenses these modified units into carbon quantum dots. The paper's schematic surface model and simplified HOMO-LUMO energy diagram interpret the emission: the conjugated carbon core gives ultraviolet and blue bands, while surface functional groups create states that lower the energy gap and add a green component. The 20 μl HCl sample is the critical case, because its partial cleavage pattern and surface composition supposedly narrow the gap enough to make the green emission dominant.","core_discovery":"On the paper's own terms, the acidolysis phase is the decisive control point for luminescence in lignin carbon quantum dots. The authors show that HCl-driven acidolysis severs the ether linkages of spruce kraft lignin, lowers the methoxy-to-aromatic ratio through demethylation, and shifts aromatic NMR signals in ways consistent with chlorine and boron functionalization. The 20 μl HCl sample retains more bonds between aromatic units than either extreme, and its surface carries -OH, -NH2, -Cl, and B(OH)2 groups with an altered HOMO-LUMO gap. In emission, only this sample shows a strong band at 503 nm, together with a slightly longer luminescence lifetime (3.18 ns versus 2.88 and 2.71 ns); the authors attribute that green band to radiative recombination from surface states whose energy gap has been narrowed by the acidolysis-induced surface chemistry.","pith_inferences":["If the surface-state explanation is correct, chemically blocking the -NH2 or B(OH)2 groups on the 20 μl HCl sample should quench or shift the 503 nm band; pH-dependent emission measurements could test this directly.","A testable extension would be to run the same HCl-dose series on lignins with different native G/H unit ratios; if the green state requires a specific residual bond pattern, only lignins with similar architecture should reproduce it.","Comparing HCl with other acids of similar strength could separate the specific surface role of chloride from the general effect of acid-catalyzed de-etherification, which the present data do not disentangle.","The paper's interpretation would be strengthened by size-selected spectroscopy that rules out particle size and aggregation effects; without that, molecular fluorophores remain a viable alternative explanation for the green emission."],"forward_implications":["Adjusting only the HCl volume in the acidolysis step can switch the same spruce lignin precursor between blue-dominated and green-emitting carbon dots, giving a simple one-dial color-tuning route.","The green 503 nm band and the longer decay time (3.18 ns) of the 20 μl HCl sample indicate that surface states, rather than only the carbon core, control this emission.","Rising HCl lowers the Raman ID/IG ratio from 0.78 to 0.42 and increases graphitic nitrogen, so acid dosing is also a structural lever on the carbon core's degree of graphitization.","Surface dopant concentration alone does not predict emission: the 1000 μl HCl sample carries more N, B, and Cl than the 20 μl sample but emits like the no-acid sample, implying the green state depends on a specific partial acidolysis product.","The NMR evidence that aromatic functionalization occurs during acidolysis rather than during hydrothermal reaction means the synthesis's optical outcome is largely set before the hydrothermal step begins."],"supporting_citations":[{"why":"Provides the m-aminophenylboronic acid doping route and the triple-emission N,B co-doped CQD system that the synthesis adapts.","marker":"[12]"},{"why":"Supplies the hydrothermal synthesis protocol for lignin-derived nitrogen-doped carbon quantum dots used as the base method.","marker":"[11]"},{"why":"Relates lignin chemical structures to fluorescent behaviors and is used to interpret how acidolysis fragments evolve into CQDs.","marker":"[15]"},{"why":"Shows that lignin molecular weight affects CQD formation and properties, supporting the claim that acidolysis-driven depolymerization controls the final material.","marker":"[27]"},{"why":"Establishes that electron-donating substituents such as -NH2 and -OH enhance CQD luminescence, the background for the surface-group argument.","marker":"[14]"},{"why":"Provides a green-emitting nitrogen-doped CQD from alkali lignin and is the comparison point for Raman D/G bands and green emission.","marker":"[18]"},{"why":"Supports the link between acid treatment, increased graphitization, and the observed decrease in the ID/IG ratio.","marker":"[23]"}],"fun_headline_variants":["Acid dose flips spruce lignin dots to green","20 μL HCl shifts lignin dots to 503 nm glow","Tiny acid tune turns lignin carbon dots green","Hydrothermal acid dose controls dot emission","One acid spike makes lignin dots emit green"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the green 503 nm emission of the 20 μl HCl sample arises from surface chemical groups narrowing the emission energy gap, rather than from differences in particle size, clumping, or leftover small fluorescent molecules.","fun_headline_variants_meta":{"raw":{"variants":["Acid dose flips spruce lignin dots to green","20 μL HCl shifts lignin dots to 503 nm glow","Tiny acid tune turns lignin carbon dots green","Hydrothermal acid dose controls dot emission","One acid spike makes lignin dots emit green"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000478,"raw_usage":{"total_tokens":2327,"prompt_tokens":863,"completion_tokens":1464,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":479,"completion_tokens_details":{"reasoning_tokens":1393}},"tokens_in":479,"tokens_out":1464,"duration_ms":10919,"temperature":1.0,"reasoning_tokens":1393,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T21:00:59.242573+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Fractionate the 20 μl HCl sample by size-exclusion chromatography or stepwise centrifugation and measure the emission of each fraction: if the 503 nm band survives in the smallest well-dispersed particles after removal of low-molecular-weight species, the surface-state explanation is supported; if it disappears or shifts with aggregation state, the surface-state assignment fails.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the m-aminophenylboronic acid doping route and the triple-emission N,B co-doped CQD system that the synthesis adapts."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the hydrothermal synthesis protocol for lignin-derived nitrogen-doped carbon quantum dots used as the base method."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Relates lignin chemical structures to fluorescent behaviors and is used to interpret how acidolysis fragments evolve into CQDs."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows that lignin molecular weight affects CQD formation and properties, supporting the claim that acidolysis-driven depolymerization controls the final material."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes that electron-donating substituents such as -NH2 and -OH enhance CQD luminescence, the background for the surface-group argument."},{"cited_title":"Zhang, Y","cited_arxiv_id":null,"evidence_quote":"Provides a green-emitting nitrogen-doped CQD from alkali lignin and is the comparison point for Raman D/G bands and green emission."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supports the link between acid treatment, increased graphitization, and the observed decrease in the ID/IG ratio."}],"review_version":1}