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 fluorescent probe, HNT, was synthesized by 1-hydrazine methyl-2-naphthol and 2,3,4-trihydroxybenzaldehyde. CN

 has stronger basicity and a superior hydrogen-bonding ability than other anions. Therefore, CN

 can deprotonate naphthalene hydroxyl in fluorescent probe HNT, which induced the bright yellow fluorescence that was quenched before changing the color of the solution from yellow to orange. This color change indicates that fluorescent probe HNT showed selective recognition of CN

 and was not interfered with by other anions. HNT can quantitatively detect CN

 M. The manufactured HNT test strip could also quickly detect CN

, which indicates that HNT has certain application value.

Cyanogen has attracted extensive attention because of its outstanding virulence and practicability among many anions. The toxicity of CN

 can cause serious harm to the environment and living organisms (1). When living organisms ingest a certain amount of CN

 through respiration, their digestive tract, or the surface of their skin, CN

 will inhibit the activity of various enzymes in cells, causing symptoms such as vomiting, convulsions, and even central respiratory failure and death (2–4). Cyanide is also widely used in industrial production to synthesize polymers fibers and resins, extract gold, and synthesize various herbicides, which increases the possibility of life being harmed by CN

 (5–7). The United States and the World Health Organization (WHO) stipulates that the maximum content of CN

 in drinking water should not exceed 1.9 μmol/L (8). Given this regulation, the synthesis of a fluorescent probe with a high sensitivity, low manufacturing cost, short response time, and specific recognition of CN

, is significant to the environment and organisms. In the literature, probes mainly detect CN

 through mechanisms such as addition, hydrogen bonding, and deprotonation (9–13). Among them, deprotonation uses CN with strong basicity to undergo deprotonation reaction with the hydroxyl or imino group in the probe to change the electron distribution in the probe molecule and generate corresponding photoelectrochemical changes. Currently, the probe is the most effective method for detecting CN

 concentration (14–18). Our team is also very interested in such probes (19–28).

In this work, 2-hydroxy-1-naphthaldehyde derivatives were used as luminescent groups because of their good spectral characteristics (14,29,30), which improves the sensitivity of these probes. To ensure the probe has better selectivity and recyclability, we designed a probe based on naphthaldehyde (31–33). After the probe reacted with CN

, the molecule was deprotonated because of the strong basicity of CN

. The fluorescence and UV spectrum of the molecule changed simultaneously, the color of the solution changed from yellow to orange, and the fluorescence was quenched at λ

= 365 nm. The results show that the HNT flourescence probe can selectively recognize CN

. The detection limit of the HNT probe for cyanide has been as low as 6.93 × 10

 M. In addition, we have made a test strip for the HNT probe, which can quickly identify and detect the contents of water samples. According to the mechanism of the above process, we verified it by 

H nuclear magnetic resonance (NMR), UV-vis, and other different methods. Besides that, we also made density functional theory (DFT) calculations.

All reagents and solutions are commercially available and analytically pure without further purification. The experimental water was double distilled and prepared by a standard method. The tetrabutyl ammonium salt (F

) of the anion were analytically pure and purchased from Alfa-Aesar Chemical Reagent Co. and stored in a vacuum dryer.

UV-vis spectra were measured on a Shimadzu UV-2550 UV spectrometer. The fluorescence emission spectrum was recorded on a Shimadzu RF-5301 fluorescence spectrometer; 

H NMR used tetramethylsilane (TMS, δ grade) as the internal standard and was measured at 600 MHz on a Mercury-400BB spectrometer. The melting point was measured on the X-4 digital micro melting point meter. Electrospray ionization–mass spectrometry (ESI-MS) was measured on a ZAB-HS instrument.

Synthesis and Characterization of Probe HNT

The intermediate 1-hydrazine methyl-2-naphthol was synthesized according to the method in the literature (34). 2-hydroxy-1-naphthalaldehyde hydrazine (0.9309 mg, 5 mM) and 2,3,4-trihydroxybenzaldehyde (0.7706 mg, 5 mM) were added to a round bottom flask with dimethylformamide (DMF) (25 mL) as the solvent and refluxed under stirring at 120 °C for 7 h before being cooled to room temperature, filtered to obtain a yellow solid, and crystallized with water ethanol to obtain a pure product (Scheme 1). The yield was 72% (m.p. > 300 °C). 

H-NMR spectral conditions and results are as follows: H-NMR (600 MHz, DMSO-d

H). IR(KBr; cm−): 3442 (s), 1625 (s), 1390 (m), 1353 (m), 1320 (m), 1177 (m), 993 (w), 840 (w), 786 (m), 742 (w); 

SCHEME 1: The synthetic procedure for sensor HNT.

The synthesis steps of fluorescence probe HNT were introduced in detail, and its structure has been characterized by 1H NMR, infrared (IR) spectroscopy, and ESI–MS. Next, the influence of different anions (F

 M) on the spectral changes of chemical probe were studied by UV spectroscopy and fluorescence spectroscopy.

After adding 50 equivalents of different anions (F

), a dimethyl sulfoxide (DMSO):water (1:1, v:v) solution containing the fluorescence probe HNT was utilized as the host to fully react and observe the changes of the solution. The results showed that the color of the solution containing the HNT probe changed from bright yellow to orange only after CN

 was added. At the same time, the corresponding absorption spectrum data showed that the strong absorption peak at 375 nm disappeared, and a new absorption peak appeared at 428 nm. However, other anions (F

) did not cause any significant change in the absorption spectrum of the HNT solution. (Figure 1a).

FIGURE 1: (a) Absorbance luminosity change spectrum of various anions added to sensor HNT solution (2.0 × 10

 M). Inset: Photographs of other anions and CN

 color changes added to HNT solution under visible light. (b) The fluorescence intensity change spectrum of various anions added to sensor HNT solution (2.0 × 10

 color changes added to HNT solution under 365 nm light.

The dimethyl sulfoxide:water (1:1, v:v) solution containing the host HNT was excited at 365 nm of the UV lamp. The emission peak at 533 nm was significantly reduced only when the CN

 ions were added (Figure 1b). Under the 365 nm of lamp, the solution was observed to change from bright yellow to colorless. When other anions were present in solution, the fluorescence color and emission intensity of the host HNT had no obvious change.

To verify the anti-interference ability of the probe HNT, we investigated the recognition ability of the probe HNT to CN

) by recording the UV-vis spectrum and fluorescence spectrum. As shown in Figure 2a, there was no obvious photochemical change in dimethyl sulfoxide:water (1:1, v:v) solution with other anions and host HNT. When CN

 was added to each group of solutions, the UV-vis spectrum and emission intensity were similar to the solutions, which contain CN

 only. The results showed that the existence of other competitive ions had an inconsiderable effect on the specific detection of CN

FIGURE 2: (a) Absorbance spectrum change diagram of adding 20 equivalents of various anions and 20 equivalents of CN

 = 428 nm). (b) The fluorescence intensity change diagram of adding 20 equiv. of various cations and 20 equivalents of CN

 was further studied by UV-vis and fluorescence titration. As shown in Figure 3a, once the concentration of CN

 increased from 0.0 to 8.2 equivalents, the absorption peak at 375 nm decreased and at 428 nm it increased gradually. With the increased CN

 concentration, the emission intensity at 533 nm decreased gradually under a 365 nm excitation wave. When the concentration of CN

 increases to 12.4 equivalents, the fluorescence emission intensity is the weakest (Figure 3b).

FIGURE 3: (a) Absorption spectrum of HNT (2.0 × 10

O (1:1, v/v) solution. Inset: Diagram of the relationship between absorbance and the number of equivalents of CN

. (b) Fluorescence spectra of HNT (2.0 × 10

 are acquired in dimethyl sulfoxide:water (1:1, v/v) solution. Inset: Diagram of the relationship between fluorescence intensity and the number of equivalents of CN

According to fluorescence titration spectrum, the changes in the fluorescence intensity of HNT with CN

 were almost linear (Figure 4). We calculated the fluorescence detection limit of the HNT probe for CN

 is the standard deviation of blank solution). The results showed that the detection limit of the fluorescence spectrum was 6.93 × 10

 M for cyanide, which was far lower than the maximum level of cyanide in drinking water from the WHO guidelines. Using the same calculation formula, the detection limit of UV-vis titration is 2.73 × 10

 M. Compared with the recently reported cyanide fluorescent probe, HNT could detect cyanide with lower concentration by UV and fluorescence methods (Table I).

FIGURE 4: Fluorescence detection limit of HTN on CN

 in dimethyl sulfoxide:water (1:1, v/v) solutions.

To further study the applicable range of probe HNT when mixed with CN

, we used fluorescence spectrometry. Using fluorescence spectrometry allowed us to study the effect of pH on the recognition process. CN

 was added to the HNT buffer solution at different pH levels, and its fluorescence emission intensity was measured. As shown in Figure 5, the probe HNT showed significant fluorescence reduction to CN

 in the pH range of 2.0–9.0, and there was no obvious change in other working ranges. This result indicated that pH has some influence on the recognition of CN

 in the dimethyl sulfoxide:water (v:v, 1:1) solution.

We further discussed the recognition mechanism of probe molecules to CN

H NMR titration and DFT calculation: Through 

H NMR titration experiments, we can see that the host molecules showed characteristic structural changes after CN

 was added to the solution of host HNT. As shown in Figure 6, the host HNT molecule has a strong peak corresponding to naphthalene hydroxyl at 12.48 ppm. When 1 equivalent of CN

) was added to the host molecule in total, the -OH peak at 12.48 ppm disappears. Because the strong basicity of CN

, a deprotonation reaction with strong acid is caused easily. Moreover, through 

H NMR spectra, we found that the position of aromatic peak gradually moved to the up field with the increase of CN

, electron delocalization and intramolecular charge transfer occurred inside the host HNT.

H NMR spectra of HNT in dimethyl sulfoxide upon addition of CN

To further explore the reaction mechanism of HNT and CN

, DFT calculation was carried out and analysis was carried out according to B3LYP/6-31G theory. As shown in Figure 7, it showed that the highest unoccupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of HNT-related molecules were almost distributed on the naphthalene ring. After adding CN

, the hydroxyl group in the naphthalene ring was deprotonated, and the electrons in LUMO were transferred to the benzene ring of 2,3,4-trihydroxybenzaldehyde. The energy gaps between them is 0.15452 au and 0.08831 au, respectively. The energy gaps indicated that the energy gap decreases and the system is more stable after the interaction between HNT and CN

H-NMR titration and DFT results, a possible recognition mechanism of the system was carried out by the Scheme 2.

FIGURE 7: The DFT calculates a graph of LUMOs, HOMOs, and energy gap for HNT and HNT with CN

Scheme 2: Proposed binding mode of chemosensor HNT with anions in the solution.

We tested the reversibility of detection and recognition of CN

 by adding hydrochloric acid to the mixed solution of HNT and CN

. As shown in Figure 8, the fluorescence intensity of the HNT solution with CN

 was marked as “Off”, and the fluorescence emission intensity was significantly reduced at 533 nm. When concentrated hydrochloric acid was added to solution of HNT and CN

, the fluorescence emission intensity was significantly enhanced, and the labeling phenomenon was labeled as “On.” After adding CN

 continuously, the mixed solution showed a decrease in fluorescence intensity, which indicated that the HNT has reversibility when recognizing and detecting CN

. In repeated operations, the phenomenon of “Off–On–Off” was repeated many times with almost no fluorescence loss.

 alternately in a dimethyl sulfoxide:water v/v = 1:1) solution of HNT-CN

, the reversibility of the sensor HNT was achieved (λ

To verify the recognition and detection of CN

 by host HNT in actual environment, we have made a test paper based on host HNT. Test strips that were 4 cm long and 1 cm wide were prepared, soaked in a dimethyl sulfoxide solution of main HNT (0.01 mol/L) completely, and then dried in a vacuum drying oven. As shown in Figure 9, when the CN

 reserve liquid is dripped onto the test strip, yellow color of the test strip changed to orange, and the yellow color was disappeared under the UV lamp. The above phenomenon proved that the test strips were used for rapid detection of CN

FIGURE 9: Images of HNT on test papers under irradiation at 365 nm. (a) Only HNT, (b) after immersion in DMSO:H

O (v:v, 1:1) solution with other anions, and (d) after immersion in a DMSO:H

Based on the above properties, we used the photochromic ability of HNT under UV-vis light stimulation to design a logic circuit containing input and output signals (38). The logic circuit was designed with four types of input signals: (In1: 254 nm UV light; In2: >500 nm visible light; In3: CN; and In4: HCl) and output signal (OUT1: emission intensity at 533 nm). The “open” state is assigned the boolean value “1”, and the “closed” state is assigned the boolean value “0”. When the emission intensity at 533 nm drops to the lowest, the output signal is regarded as an open state, the boolean value is “1”. The situation is regarded as closed, and the boolean value is “0”. The logic circuit diagram corresponding to the truth table is shown in Figure 10.

FIGURE 10: (a) Truth table for the logic circuit; (b) Implication logic gates represented by IEEE-recommended symbols.

In this paper, we designed and synthesized a new type of high-sensitivity sensor HNT, which could colorimetric and fluorescent response to CN

 in aqueous solution, and the minimum detection limit is 6.93 × 10

 M. Fortunately, the fluorescence of HNT changes after adding CN

 and AcOH, so it can be applied to the ultra-sensitive Implication logic circuit. In addition, we have produced test strips that were used to quickly and easily detect CN

. Based on the various properties and applications of probe HNT, it was concluded that probe HNT is a promising CN

 indicator with strong anti-interference abilities, easy operation, fast response, and low detection limit.

We gratefully acknowledge the support of the National Nature Science Foundation of China (No. 21467012).

(1) R. Ali, S. K. Dwivedi, H. Mishra, and A. Misra, 

(2) S. Ahmadi, H.A. Dabbagh, P. Grieco, and S. Balalaie, 

Spectrochim Acta - Part A Mol Biomol Spectrosc.

(3) Ö. Şahin, Ü.Ö. Özdemir, N. Seferoğlu, B. Aydıner, M. Sarı, and T. Tunç, 

(4) C.B. Bai, X.Y. Liu, J. Zhang, R. Qiao, K. Dang, and C. Wang, 

(5) A. Mondal, A. Hazra, J. Chakrabarty, J.C. Bose, and P. Banerjee, 

(6) M.S. Kim, D. Yun, J.B. Chae, H. So, H. Lee, and K. Kim, 

(8) X.S. Shang, N.T. Li, Z.Q. Guo, and P.N. Liu, 

(9) J.M. Jung, D. Yun, H. Lee, K.T. Kim, and C. Kim, 

Spectrochim Acta - Part A Mol Biomol Spectrosc. 

(11) J. Chao, M. Xu, Y. Liu, Y. Zhang, F. Huo, and C. Yin, 

(13) X.S. Shang, N.T. Li, Z.Q. Guo, and P.N. Liu, 

(14) X.Q. Ma, Y. Wang, T.B. Wei, L.H. Qi, X.M. Jiang, and J.D. Ding, 

(15) G.F. Gong, Y.Y. Chen, Y.M. Zhang, Y.Q. Fan, Q. Zhou, and H.L. Yang, 

(16) Q Zhao, G.F. Gong, H.L. Yang, Q.P. Zhang, H. Yao, and Y.M. Zhang, 

(17) H.H. Yang, P.P. Liu, J.P. Hu, H. Fang, Q. Lin, and Y. Hong, 

(18) W.J. Qu, H.H. Yang, J.P. Hu, P. Qin, X.X. Zhao, and Q. Lin, 

(19) C. Long, J.H. Hu, Q.Q. Fu, and P.W. Ni, 

(20) P.W. Ni, Y. Yao, Q.Q. Fu, C. Long, and J.H. Hu, 

(22) Q.Q. Fu, J.H. Hu, Y. Yao, Z.Y. Yin, K. Gui, and N. Xu, 

(23) J.H. Hu, C. Long, Q.Q. Fu, P.W. Ni, and Z.Y. Yin, 

(24) J.H. Hu, Y. Sun, J. Qi, Q. Li, and T.B. Wei, 

Spectrochim. Acta - Part A Mol. Biomol. Spectrosc.

(25) Z.Y. Yin, J.H. Hu, K. Gui, Q.Q. Fu, Y. Yao, and F.L. Zhou, 

(26) P.X. Pei, J.H. Hu, Y. Chen, Y. Sun, and J. Qi, 

(27) P.X. Pei, J.H. Hu, C. Long, and P.W. Ni, 

Spectrochim Acta - Part A Mol. Biomol. Spectrosc.

(28) J.H. Hu, J.B. Li, Y. Sun, P.X. Pei, and J. Qi, 

(29) J.H. Kang, S.Y. Lee, H.M. Ahn, and C. Kim, 

(30) X.Q. Ma, Y. Wang, T.B. Wei, L.H. Qi, X.M. Jiang, and J.D. Ding, 

(31) L. Patra, K. Aich, S. Gharami, and T.K. Mondal, 

(32) P. Ravichandiran, A. Boguszewska-Czubara, M. Masłyk, A.P. Bella, P.M. Johnson, and S.A. Subramaniyan, 

(33) K. Rezaeian, H. Khanmohammadi, and A. Talebbaigy, 

(34) D. Roy, C. Ahakraborty, and R. Ghosh, 

(35) W. Wang, M. Wu, H. Liu, Q. Liu, Y. Gao, and B. Zhao, 

(37) R.V. Rathod, S. Bera, and D. Mondal, 

Spectrochim Acta - Part A Mol. Biomol. Spectrosc

(38) G. Sun, W. Chen, Y. Liu, X. Jin, Z. Zhang, and J. Su, 

 are with the College of Chemical and Biological Engineering at Lanzhou Jiaotong University, in Lanzhou, China. Correspondence should be addressed to Jing-Han Hu at the following e-mail address: 

Articles

A novel hydrazone-based CN- fluorescent probe, HNT, is described here, and a study was done to see if it could show selective recognition of CN- ions.

A Highly Selective Colorimetric and “Turn-Off” Fluorimetric Probe Based on Unsymmetrical Naphthalene Formaldehyde Derivatives for the Detection of CN− in Aqueous Media

In this study, we present a methodical qualitative and quantitative analysis of H-Classic publications that have made key contributions and identify intensive mainstream research areas in atomic spectroscopy. The main objective of this study is to determine which publications are cited the most and who contributed to those publications. This study also provides an insight into recent historical developments in atomic spectroscopy, which can be valuable to academic organizations and editorial staff in determining areas and fields of research. This study also creates awareness about the research trends in atomic spectroscopy among research communities, government organizations, and funding bodies.

During the last decade, there has been immense research activity in atomic spectroscopy. However, there is no methodical analysis currently available in the literature that examines the most cited publications, their trends, and their characteristics. Studies such as this one identify and document the articles that are most cited by researchers and have made key contributions. For the set of identified publications, data are extracted to obtain the number of publications appearing in each year as well as their citation characteristics, which helps determine the evolution of any particular research field.

Bibliometric studies have been conducted in many fields, including the internet of things (1), cybernetics (2), cloud computing (3), software engineering (4), intelligent transportation systems (5), soft computing (6), enterprise architecture research (7), big data for medical research, supply chain management, business intelligence, (8–12), fuzzy decision research, (13–15), 5G (16), limb prosthetics (17), cleft lip and palate (18), hypospadiology (19), craniofacial surgery (20), neuroscience (21), pelvic floor physiotherapy (22), orthopedic elbow surgery (23), rheumatology (24), craniofacial anomalies and craniofacial surgery (20), insular cortex brain (25), osteoporosis (26), health care disparities (27), regenerative endodontics (28), and telemedicine (29).

The main aim and objective of this study is to determine who contributed to the most highly cited publications in the field of atomic spectroscopy. Along with identifying contributors, the study also analyzes factors, such as citation counts and publication dates. These research insights and trends can assist researchers in looking at the current ranking of scientific contributions to refine their research plans and focus on a particular research area. These research insights also help determine potential project funding, assess research proposals, decide promotion cases of researchers to higher ranks, and create awareness among research communities, government organizations, and funding bodies. The outcome of this study answers the following research questions:

the volume, scope, and spatial distribution of research productivity in the literature

statistical analysis of H-Classics papers published with impact factors

most prolific authors, institutes, funding agencies, and countries in terms of quantity and total citations of publications

analysis of top research topics and keywords

research collaborations among authors, countries, institutions, and journals

Electromagnetic radiation carries energy in waves. When these waves strike any material or particle, they produce a change in the wave, the particle, or both. The study of the effects of interaction between electromagnetic radiation and matter is called 

 is a quantitative analysis method for measuring the elemental composition of matter at low levels and with high accuracy. The detection limit varies from 100 to less than 0.001 parts per billion (ppb, μg/L), for multiple elements, depending on the atomic spectroscopy method applied.

Atomic spectroscopy is extensively used to study the internal structure of matter, especially molecules of complex organic or inorganic compounds. Atomic spectroscopy helps determine the level of micronutrients in plants and animals. It also traces elements in soil, polluted water, and living things. Deficiency or increase levels of essential trace elements in our diets has led to severe health problems. As an example, metals, such as mercury, lead, and cadmium, are toxic to the human body (30). As a result, relevant applications for atomic spectroscopy include food and water analysis for element quantity, pharmaceuticals (for the remain catalyst in final products), and biological tissues (blood, liver, and brain muscle tissues) for elemental composition.

With each passing day, demand for atomic spectroscopy systems is increasing because of more stringent industry requirements. To ensure safety and quality regulations, industries demand that before their products are launched into the market, that their products comply with all safety and quality standards. Key driving factors include drug safety, fluorescence, and food safety. In terms of global market trends and investments, atomic spectroscopy as a field is projected to substantially increase as an industry to above the $6.5 billion mark by the year 2024 (31).

Using citation analysis, the study of atomic spectroscopy provides an insight into the recent historical developments of highly cited papers in the field through the use of H-Classics, which can be valuable to any academic organization and publications editorial staff in understanding the scientific structure and determining subfields of research. Citation analysis helps look at the structure, dynamics, and hierarchy of research, and it is a measure of influence, effect, and impact, irrespective of publication counts. Citations reveal useful information about scientific progress in the field. They are used to determine publication’s quality, their impact, and emerging research fronts in the subject area.

The next section of this article elaborates on the approach used to analyze the data. The section following that provides a detailed analysis and result outcomes before finally drawing conclusions on the research that has been compiled.

Bibliometric analysis is used to analyze and manage citations of research trends for any particular research field (32). As with any statistical data, bibliometric analysis is most effective when analyzed properly. Bibliometric analysis helps determine which papers are most popular in the academic community. It also helps in determining which research is having an influence in its field. The immense breadth and depth of data lead to certain challenges as publishing and citation data vary across categories and publications, which eventually results in getting a broader picture of performances in relevant fields.

As a rule of thumb, publications in different disciplines are cited at a different pace or have different citation behavior. Consider the life sciences field. In life sciences, papers initially may garner a lot of citations, but will quickly taper off, whereas in mathematics, by comparison, the citation counts are lower, but the same papers will continue to gain new citations for years after publication. Based on these trends, it would be unfair to expect a mathematics department to achieve the same rate of citation as a biology department.

Therefore, institutions must find a way to fairly measure the performance of different disciplines. Older material tends to be more highly cited because it simply had more opportunity (time) to be cited. The age of publications complicates the process of comparing publications to one another. Researchers who have been publishing for a long time are more likely to have a greater number of publications and more citations than an author who is new; a direct comparison of the two would be biased in favor of the earlier researcher in this case. Citation distribution tends to be quite uneven, with only a small number of publications attaining the most cited status. Many publications have few or no citations. Citation count also does not indicate anything about the quality of the research that was published. Researchers might be citing the work because they disagree with it. Therefore, it is more suitable to mitigate these challenges by using relative measures.

Citation Classics was introduced by Eugene Garfield in 1964 (33). It was designed for information retrieval to help researchers find what they need in the literature. In the scientific community, Citation Classics is a common practice that is employed to determine the quality of a publication, its impact, and the emerging research fronts in a particular subject area. Tracking and examining the number of citations an article receives helps establish links to other research works. Some use it as a quantitative measure, related to research quality and quantity. Citation Classics helps in looking at the structure, dynamics, and hierarchy of research and is a measure of influence, effect, and impact, irrespective of publication counts. When publications are frequently cited, it is an indication that they withstand the test of time and are particularly noteworthy in their field. This concept of identifying citation classics had selection criteria with two approaches: The first approach is setting thresholds of citation rates (to put it simply, authors set 

 number of citations without any significant scientific backing), and the second approach is setting a threshold value from the list of published papers sorted (in descending order) by the number of citations that are received.

The second approach had a limitation of not taking into consideration the scientific progress of research areas or their citation pattern. A new concept of H-Classic was presented (34) to address this issue, where variable or adaptable thresholds are used for each of the different research areas. This approach provides an unbiased criterion for identifying citation classics based on the systematic and scientific methods in any field of research. H-Classics is based on the famous H-Index, proposed by George Hirsh in 2005. H-Index measures how consistently a work is cited. It is an index that aims to measure both the productivity and impact of a set of publications. It is the most widely used quantitative measure of productivity (from number of publications), citation-based impacts, and number of citations simultaneously. By combining them into a single number, this reduces the artificial influence of citation counts (one or two highly cited papers). H-Index is commonly associated with a set of publications. The H-Index is determined by finding out the 

 number of articles in a set of publications that have been cited 

 is the largest possible number for the set of publications. The details of H- Index is discussed further in this article.

The impact factor (IF) of a journal is a well-known measure of how often the articles in journals are cited by later articles. IF considers articles published in a journal over the previous two years. One classic definition of IF is the ratio of citations received by all items published in that journal during the last two-year period to the number of articles published in the last 

 year period. The impact factor score indicates how highly cited an average article published in a particular journal is relative to other articles in its field or discipline. Comparing IFs of journals in the same field gives a rough indication of which journals are more influential. However, IF is not an indicator of the quality of the journal. Journal Citation Reports (JCR) is a resource, which is released annually by Clarivate Analytics, for understanding the citation patterns in different subject categories. It helps in evaluating journal quality based on impact factor.

Different scientific fields have different patterns of citation activity. It is important to note that if one journal scores high in one subject category, it might differ from what would be considered a high score in another subject category. In other words, the range of possible values varies by subject categories. In addition to impact factor score, JCR also publishes quartile rankings that are derived for each journal in each of its subject categories based on the IF distribution such as Q1 (top 25%), Q2 (between top 50% to top 25%), Q3 (top 75% to top 50%), and Q4 (bottom 25%).

The identification of classic articles, from any of the research area, through H-Classis is carried as follows.

Data Sources - Bibliographic Database for Study

Web of Science (WOS), Scopus, and Google Scholar are available and popular databases. These are widely used to search and study scholarly work in relevant fields. However, Scopus analyzes data primarily only from 1996 onwards. Google Scholar has larger citations indexed, which include peer-reviewed journals, and also books, monographs, thesis dissertations, and technical reports. Because grey literature is also included, Google Scholar citation information may not be considered to be very accurate or reliable. Therefore, in our study, we chose to use WOS because it is the most effective, adequate, and comprehensive multidisciplinary database. It has strong coverage and maintains records from 1900 onwards. WOS is renowned as one of the largest citation index databases, with over 90 million records in over 55 disciplines and over 800 million cited references, maintained by Clarivate Analytics. The database includes the Science Citation Index Expanded (1900–present), Social Sciences Citation Index (1900–present), Arts & Humanities Citation Index (1975–present), Conference Proceedings Citation Index – Science (1998–present), and Conference Proceedings Citation Index – Social Science & Humanities (1998–present).

For such bibliometric studies, only articles and review papers are considered “certified knowledge” (35), apart from books, thesis, discussions, news items, meeting abstracts, and notes. Therefore, in our study of citation classics, we selected papers only of two document types—articles and reviews. In the case of WOS, research areas are already mapped and matched with areas of JCR, so the effort for retrieving relevant documents individually is reduced.

Tools are available to facilitate the computation of H-Index automatically, but H-Index can also be computed manually. In order to compute H-Index, the available dataset should be listed, ranked, and sorted in descending order by the number of citations received. This process brings the paper with the highest numbers of citations to the top. The H-Index is the paper order, immediately above the first paper whose order is just below its citation count.

The top highly cited papers would be included in the H-Core of research area. H-Core of research area consists of H-Classics, so the H-index of a research area symbolizes the cardinality of the H-core of the research area. The H-Core of the research area includes the first highly cited papers from the previous ranking, meaning the H-Classics and thus the H-Index of a research area is the ranked H-Core.

Setting of Research Data Under Study & Sample Size

The source of data presented in this study was collected in April 2020, from the Science Citation Index – Expanded (SCI-Expanded). In WOS, the field tag “TS” finds records containing the specific “term” in abstracts, titles, and keywords fields. The following advanced search query was used, which includes name variations in spellings or abbreviations to retrieve and extract papers from WOS (by Clarivate). Such data adequately reflects the literature in the field. A query below was executed to retrieve all necessary detailed information from bibliographic database to analyze the data.

The search query resulted in 1214 documents. Out these, 801 were articles and 111 were reviews, which were considered. All papers were in the English language. Documents were sorted in descending order based on citation counts. The H-Index, for the available dataset, was found to be 64. Therefore, the first 64 documents were classified as highly cited papers based on the H-Classic definition were considered for further analysis. The year-wise distribution ranged from 1968 to 2015.

This section discusses the results and outcomes of the study. We begin by identifying H-Classic highly cited papers with the year of publication, impact factor, quartile rankings, and total and average citations. This step is followed by co-citation analysis and network visualization. Finally, the contributions of authors, organizations, and countries (or territories) are presented.

H-Classics – Highly Cited Papers Details

As mentioned earlier, 64 highly cited papers, based on the concept of H-Classics, were investigated. These 64 H-Classics publications were contributed by 249 authors from 110 institutions and distributed among 25 countries or territories.

The earliest H-Classic article, “Some Applications of Microwave-Excited Electrodeless Discharge Tubes in Atomic Spectroscopy”, was published in 1968 in Applied Optics.

Figure 1 illustrates the distribution of H-Classics publications per year between 1968 and 2015. It shows the overall trend according to the annual number of H-Classic papers in atomic spectroscopy. Starting in the year 1968 with one paper, followed by three papers between 1980–1984, five papers between 1985–1989, 10 papers between 1990–1994, nine papers between 1995–1999, 15 papers between 2000–2004, 12 papers between 2005–2009, seven papers between 2009–2010, and two papers in 2015. It is interesting to note that none of the H-Classic papers were published before 1968 and no publications have yet been classified as H-Classic since 2015. Year 1990 to 2009 had a large number of publications, and the year 2005 had the most with six papers. Since 2005, the number of H-Classics publications has decreased.

FIGURE 1: Publication trend of H-classics per year; abscissa is the year and ordinate is the number of publications.

Table II shows the breakdown of the 64 H-classics with authors, titles, sources, quartile rankings, impact factor, total citation count, average citation count (citation count per year since the document was published), and average citation count of the first 10 years since publication. Table II includes the ranking by total citation count (Rank TC), average citation count (Rank AC), and average citation count of the first 10 years (Rank A10C). Currently, it is sorted based on Rank TC.

Amongst all 64 H-Classics, a paper authored by Mitchell and others received the maximum citation count of 503. The paper was published in 

 in 2004, which focused on super-resolving phase measurements with a multiphoton entangled state. A subsequent paper, with maximum citation count of 439, was by J.P. Toennies and others that focused on spectroscopy of atoms and molecules in liquid helium. That paper was published in 1998 (36).

In terms of average citation count (citation count per year since it was published), the paper that ranked at the top was published by Ushijima and others. The article focused on cryogenic optical lattice clocks and was published in Nature Photonics (37). This paper ranked fourth in terms of total citation count (307). It is interesting to note that paper (38) ranked seventh in terms of average citation count (

In this study, a methodical qualitative and quantitative analysis of H-Classic publications that have made key contributions and identify intensive mainstream research areas in atomic spectroscopy is presented. 