Novel biomass-derived carbon dots co-doped with nitrogen and sulfur were fabricated through facile and simple synthetic method from manufactured milk powder and methionine as precursors. The as-fabricated platform was used for ratiometric fluorescence sensing of Cu (II) and bisphosphonate drug risedronate sodium. The sensing platform is based on oxidation of o-phenylenediamine by Cu (II) to form 2, 3-diaminophenazine (oxidized product) with an emission peak at 557 nm. The resultant product quenched the fluorescence emission of as-fabricated carbon dots at 470 nm through Förster resonance energy transfer (FRET) and inner-filter effect (IFE). Upon addition of risedronate sodium, the formation of 2, 3-diaminophenazine was decreased as a result of Cu (II) chelation with risedronate sodium, recovering the fluorescence emission of carbon dots. The ratio of fluorescence at 470 nm and 557 nm was measured as a function of Cu (II) and risedronate sodium concentrations. The proposed sensing platform sensitively detected Cu (II) and risedronate sodium in the range of 0.01–55 μM and 5.02–883 µM with LODs (S/N = 3) of 0.003 μM and 1.48 µM, respectively. The sensing platform exhibited a good selectivity towards Cu (II) and risedronate sodium. The sensing system was used to determine Cu (II) and risedronate sodium in different sample matrices with recoveries % in the range of 99–103 % and 97.4–103.8 %, and RSDs % in the range of 1.5–3.0 % and 1.8–3.6 %, respectively.

A ratiometric-based fluorescence emission system was proposed for the determination of sulfide. It consists of blue emissive graphene quantum dots (GQDs) and self-assembled thiolate-protected gold nanoclusters driven by aluminum ion (Al³⁺@GSH-AuNCs). The two types of fluorophores are combined to form a ratiometric emission probe. The orange emission of Al³⁺ @GSH-AuNCs at 624 nm was quenched in the presence of sulfide ion owing to the strong affinity between sulfide and Au(I), while the blue GQDs fluorescence at 470 nm remained unaffected. Interestingly, the Al³⁺@GSH-AuNCs and GQDs were excited under the same excitation wavelength (335 nm). The response ratios (F₄₇₀/F₆₂₄) are linearly proportional to the sulfide concentration within the linear range of 0.02–200 µM under the optimal settings, with a limit of detection (S/N = 3) of 0.0064 µM. The proposed emission probe was applied to detect sulfide ions in tap water and wastewater specimens, with recoveries ranging from 95.3% to 103.3% and RSD% ranging from 2.3% to 3.4%, supporting the proposed method's accuracy.

A simple fluorescence method is described for measuring rutin dependent on the nitogen-doped carbon dots
(NCDs) prepared via simple pyrolysis method from chicken feet biowaste. The as-fabricated NCDs have unique
advantages including cost-effectiveness and high quantum yield (42.9 %). The as-prepared NCDs explore an
optimal emission band at 430 nm following exciting NCDs at 330 nm. Addition of rutin to blue-emissive NCDs
quenched their fluorescence emission by inner-filtration effect (IFE) and static quenching. Under optimized
conditions, the fluorescence responses increased as the rutin amount was raised from 100 to 1000 nmol/L with
5.3 nmol/L as a detection limit (S/N = 3). The probe selectivity was improved by adding bovine serum albumin
(BSA), which binds other structurally-related compounds (other flavonoids). The as-synthesized NCDs exhibited
some advantages towards rutin detection such as: lower LOD value, satisfactorily reproducibility, simplicity,
rapidity, selectivity, and stability. The sensing probe was efficiently utilized for determining rutin in different
real samples with acceptable results. The sensor offers an efficient biowaste-based approach for the determination
of (bio) molecules.

A highly responsive and selective coordinated molecularly imprinted polymer (MIP)-dependent ratiometric
electrochemical platform for ciprofloxacin (CIPX) detection was developed. It consists of porous carbon modified
with heteroatoms (N, S@C) built on pencil graphite electrode (PGE) surface. The N, S@C/PGE was then modified
using silver nanoparticles (AgNPs), which were employed to boost conductivity and surface area of the electrode,
as well as an internal reference signal. Molecularly imprinted film was built on AgNPS/N, S@C/PGE surface in
the existence of template (CIPX) via electro-deposition of Cu (II)-coordinated pyrrole-3-carboxylic acid. The
detection of CIPX is based on the measurement of ICIPX/IAgNPs ratios using differential pulse voltammetry (DPV),
which were linearly proportional to CIPX concentration in the range of 0.13–650 nM with LOD (S/N = 3) of 38
pM. The as-fabricated coordinated MIP-dependent ratiometric electrochemical platform exhibited many advantages
including low detection limit, long-term stability, detection reliability, and good reproducibility/
selectivity. The electrochemical probe was efficiently used for the analysis of CIPX-containing specimens with
recoveries % in the range of 97.6–101.6 % with RSDs % in the range of 2.6–3.2 %, proving accuracy of the
platform.