Mqtt publish plain format how to change to json format

//Including the four libraries
#include <UniversalTelegramBot.h>
#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <PubSubClient.h>

//------- WiFi Settings -------
const char* ssid = "";
const char* password = "";
const char* mqtt_server = "";
const char* mqtt_username = "";
const char* mqtt_password = ""; 

// ------- Telegram config --------
#define BOT_TOKEN ""  // your Bot Token (Get from Botfather)
#define CHAT_ID "" // Chat ID of where you want the message to go (You can use MyIdBot to get the chat ID)

const int gasAnalogPin = 32;
long Bot_lasttime;
int bulk_messages_mtbs = 12000; 
long lastMsg = 0;
int value = 0;
String val_str; //see last code block below use these to convert the float that you get back from DHT to a string =str
char val[50];

WiFiClientSecure espClient;
UniversalTelegramBot bot(BOT_TOKEN, espClient);

WiFiClient Client;
PubSubClient client(Client);

String ipAddress = "";

volatile bool telegramButton1PressedFlag = false;
volatile bool telegramButton2PressedFlag = false;
volatile bool telegramButton3PressedFlag = false;
volatile bool telegramButton4PressedFlag = false;



void setup_wifi() {

  delay(10);
  // We start by connecting to a WiFi network
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  randomSeed(micros());

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}

void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Create a random client ID
    String clientId = "ESP32Client-";
    clientId += String(random(0xffff), HEX);
    // Attempt to connect
    if (client.connect(clientId.c_str(), mqtt_username , mqtt_password)) {
      Serial.println("connected");
      client.publish("SmartToilet", "welcome");
      // ... and resubscribe
      client.subscribe("SmartToilet");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void setup() {
Serial.begin(115200);
pinMode(gasAnalogPin, INPUT);
setup_wifi();
client.setServer(mqtt_server, 1883);
client.setCallback(callback);
}
void sendTelegramMessage1() {
  String message = "Air was good @ 27 Mandai Estate Meeting Room!";
  if(bot.sendMessage(CHAT_ID, message, "Markdown")){
    Serial.println("TELEGRAM Message 1 Successfully sent");
  }
  telegramButton1PressedFlag = false;
}

void sendTelegramMessage2() {
  String message2 = "Air was poor, please turn on the aircondition fan @ 27 Mandai Estate Meeting Room!";
  if(bot.sendMessage(CHAT_ID, message2, "Markdown")){
    Serial.println("TELEGRAM Message 2 Successfully sent");
  }
  telegramButton2PressedFlag = false;
}

void sendTelegramMessage3() {
  String message3 = "Air was bad, please leave the room @ 27 Mandai Estate Meeting Room!";
  if(bot.sendMessage(CHAT_ID, message3, "Markdown")){
    Serial.println("TELEGRAM Message 3 Successfully sent");
  }
  telegramButton3PressedFlag = false;
}

void sendTelegramMessage4() {
  String message4 = "Air was terrible, please emergency evaculation @ 27 Mandai Estate Meeting Room!";
  if(bot.sendMessage(CHAT_ID, message4, "Markdown")){
    Serial.println("TELEGRAM Message 4 Successfully sent");
  }
  telegramButton4PressedFlag = false;
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
  float myfloat;
  float V0 = 750;
  float Rs = 0;
  float R0 = 0; 
  float Voltage;
  float Vout = 1183;
  float ratio = 0;
  static bool value_reach = false;
  static bool value_reach2 = false;
           R0 = ((20000*5000/V0)-40000);
           Rs = ((20000*5000/analogRead(gasAnalogPin))-40000);
  ratio = Rs/R0; //Replace R0 with the value found using the sketch above
  
    Serial.println(R0);
    Serial.println(Rs);
    Serial.println(ratio); // How to calculate PPM?
    Serial.println("ratio");
    delay(1000);
  
  long now = millis(); 
  if (now - lastMsg > 60000) {
    lastMsg = now;
    ++value;
  val_str = String(ratio); //converting ftemp (the float variable above) to a string 
  val_str.toCharArray(val, val_str.length() + 1); //packaging up the data to publish to mqtt whoa...
  if (ratio > 1.00) {                  //good stage 1
       Serial.println("Publish MQTT Message: ");
       client.publish("SmartToilet", val);  
       Serial.println("checking ratio (1) again");     
       delay(3000);  
       if(!value_reach)
{
                    if(ratio > 1.00){                                                                        
                         Serial.println("Send notification 1 to Telegram");    
                         sendTelegramMessage1();             
                         value_reach = true;
                         Bot_lasttime = 0;
             
}  
}
} else if (ratio > 0.50 && ratio < 1.00) {        //bad   stage 2
            Serial.print("Publish MQTT Message: ");
            client.publish("SmartToilet", val);   
            Serial.println("checking ratio (2) again");     
            delay(3000);
            if(value_reach)
{              
                         if(ratio > 0.50 && ratio < 1.00){  
                              Serial.println("Send notification 2 to Telegram");
                              sendTelegramMessage2(); 
                              Bot_lasttime = 0;              
            value_reach = false;  
}
}
} else if (ratio > 0.10 && ratio <0.50){        //poor  stage 3
            Serial.print("Publish MQTT Message: ");
            client.publish("SmartToilet", val);
            Serial.println("checking ratio (3) again");     
            delay(3000);
            if(!value_reach2)
{
                           if (ratio > 0.10 && ratio <0.50){           
                                 Serial.println("Send notification 3 to Telegram"); 
                                 sendTelegramMessage3();              
                                 value_reach2 = true;
                                 Bot_lasttime = 0;

}
}
} else if (ratio < 0.10){        //terrible stage 4
            Serial.print("Publish MQTT Message: ");
            client.publish("SmartToilet", val);      
            Serial.println("checking ratio (4) again");     
            delay(3000);
            if(value_reach2)
{             
                          if (ratio < 0.10){
                                Serial.println("Send notification 4 to Telegram");
                                sendTelegramMessage4();
                                Bot_lasttime = 0;
            value_reach2 = false;          
}
}
}
}
}

My MQTT now publish as plain message format , how do i change to json publish format?

If it is simple JSON strings then just construct them on the fly.

void convert2Json()
{
  // {"D":"B1","N":0,"T":24.08,"P":1019.62,"A":-52.93,"H":45.30,"V":2.96}
  packet = "";
  packet.concat(("{\"D\":\"" deviceName "\""));
  
  packet.concat((",\"N\":"));
  packet.concat(pktNumber++);
  
  packet.concat((",\"T\":"));
  packet.concat(temperature);
  
  packet.concat((",\"P\":"));
  packet.concat(pressure);
  
  packet.concat((",\"A\":"));
  packet.concat(altitude);
  
  packet.concat((",\"H\":"));
  packet.concat(humidity);
  
  packet.concat((",\"V\":"));
  packet.concat(vcc);
  packet.concat("}");
}